Enable setting headers and query parameters. Also add 'fake extensions' mechanism.

[laundmo]

This PR adds 3 features:

• Headers
• Query params
• ability to use URLs which don't end in a file extension, by adding a "fake" extension with 2 dots: ..png which will be stripped for the actual request

nice to have: example showcasing this. not sure how to do that, as the whole idea of this is to allow authentication in the requests

supersedes:

• Add field for setting the user agent #24

[laundmo]

Forgot to mention when i posted this (it was late and i was tired): I've tested this while using all the added features at the same time on both native linux and wasm in chromium.

I've also set the request mode for wasm to CORS, not sure what the default was before.

Another consideration is adding a option to set the "credentials" flag for wasm requests, which would allow passing auth headers, cookies, etc. from the tab to the request. I have no need for this and testing it is hard, so i've omitted that