Add ENS support

[yrliou]

Support the name resolution of ethereum name service (ENS) via https://resolver.cloudflare-eth.com/dns-query endpoint when the feature is enabled in brave://flags and user set the resolve method to public DoH server.
This issue is to support the same as we have for unstoppable domains in #14099 for ENS, and for .eth TLDs only.

[stephendonner]

Verified FIXED using nightly with the testplan from brave/brave-core#8125 on build

Brave	1.23.23 Chromium: 89.0.4389.72 (Official Build) nightly (x86_64)
Revision	3f345f156bfd157bd1bea06310e55f3fb2490359-refs/branch-heads/4389@{#1393}
OS	macOS Version 11.2.3 (Build 20D91)

1. Open Brave and visit brantly.eth, it should show error page saying This site can’t be reached.

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2. Go to settings, search for ens, there should be no ens domains resolve method setting.

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3. Go to brave://flags and search for unstoppable and enable the feature flag.

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4. Restart browser and go to brantly.eth should now should the interstitial for users to opt-in unstoppable domains support.

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5. Click Disable, should now see site can't be reached and Method to resolve unstoppable domains in brave://settings should now be Disabled.

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6. Change the resolve method setting back to ask and visit brantly.eth again.

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7. Opt-in interstitial should be shown again, and this time click Proceed using Cloudflare server.
8. Page should be loaded now. (Note that there will be a brief moment that it is showing site is not reached while the page is loading, this is a known and will be addressed in a new issue.)

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9. Method to resolve Ethereum Name Services (ENS) in brave://settings should now be Public DNS over HTTP server.

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10. Open a new tab and visit brantly.eth, page should be loaded normally without any interstitial pages.

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Additionally, I filed #14478 for the cropped Public DNS over HTTPS serv text in the select widget.

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Verification passed on

Brave	1.22.66 Chromium: 89.0.4389.90 (Official Build) (64-bit)
Revision	62eb262cdaae9ef819aadd778193781455ec7a49-refs/branch-heads/4389@{#1534}
OS	Ubuntu 18.04 LTS

• Verified the test plan from Add ENS support brave-core#8125

1. Open Brave and visit brantly.eth and ensured opt-in interstitial page is opened (as per the issue description Do not show opt-in page for decentralized DNS support in OTR contexts #14488 (comment))

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2. Go to settings, search for unstoppable, there is Method to resolve Ethereum Name Service ENS settings set to Ask by default

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5. Ensured that open http://brantly.eth/ in normal tab is showing up This site can’t be reached page when Method to resolve Ethereum Name Service ENS is set to Disable in brave://settings

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6,7 and 8. Set the Method to resolve Ethereum Name Service ENS is set to Ask again in brave://settings and ensured reload the page http://brantly.eth/ opens up an interstitial page and click on proceed opens up the http://brantly.eth/ page

screenshot

9. Ensured Method to resolve Ethereum Name Service ENS in brave://settings is set to Public DNS over HTTP server

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10. Ensured open a new tab and visit brantly.eth page loads normally without any interstitial pages.

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Verification passed on

Brave | 1.22.66 Chromium: 89.0.4389.90 (Official Build) (64-bit)
-- | --
Revision | 62eb262cdaae9ef819aadd778193781455ec7a49-refs/branch-heads/4389@{#1534}
OS | Windows 10 OS Version 2004 (Build 19041.867)

• Verified the test plan from Support Unstoppable Domains via DNS over HTTPS behind a feature flag brave-core#7702

1. Open Brave and visit brantly.eth and ensured opt-in interstitial page is opened (as per the issue description Do not show opt-in page for decentralized DNS support in OTR contexts #14488 (comment))

screenshot

2. Go to settings, search for unstoppable, there is Method to resolve Ethereum Name Service (ENS) settings set to Ask by default

screenshot

5. Ensured that open brantly.eth in normal tab is showing up This site can’t be reached page when Method to resolve Ethereum Name Service (ENS) is set to Disable in brave://settings

screenshot

6,7 and 8. Set the Method to resolve Ethereum Name Service (ENS) is set to Ask again in brave://settings and ensured reload the page brantly.eth opens up an interstitial page and click on proceed opens up the brantly.eth page

screenshot

9. Ensured Method to resolve Ethereum Name Service (ENS) in brave://settings is set to Public DNS over HTTP server

screenshot

10. Ensured open a new tab and visit brantly.eth page loads normally without any interstitial pages.

Encountered #14478

[zackees]

Hey I just tried to visit https://resolver.cloudflare-eth.com/dns-query and I got the following error, is this a problem?