cache NFT metadata responses

[josheleonard]

Description

Store valid NFT metadata responses within the api cache to speedup subsequent requests for NFT metadata

Steps to Reproduce

1. Create a Brave Wallet
2. Add an NFT to portfolio
3. View the NFT page
4. Navigate to a different page
5. return to the NFT page

Actual result:

The NFT meta data was not cached, preventing speedup of the second page load

Expected result:

The NFT metadata is cached, allowing for speedup on the second page load

Reproduces how often:

Brave version (brave://version info)

Version/Channel Information:

• Can you reproduce this issue with the current release?
• Can you reproduce this issue with the beta channel?
• Can you reproduce this issue with the nightly channel?

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields?
• Does the issue resolve itself when disabling Brave Rewards?
• Is the issue reproducible on the latest version of Chrome?

Miscellaneous Information:

[cypt4]

We can change policy in the APIRequestHelper for some requests. At the moment it bypasses cache. I think for erc721 tokens we could at least cache getting of metadata url. Also we could enable cache when accessing nft gateway for the metadata content.

  request->load_flags = net::LOAD_BYPASS_CACHE | net::LOAD_DISABLE_CACHE |
                        net::LOAD_DO_NOT_SAVE_COOKIES;

[nvonpentz]

Removed QA/Yes because this isn't easily verifiable by QA team.