Require X-Accept-Authenticate: LN-invoice header

Nobody has claimed this yet.

I was randomly thinking about payment protocol over HTTP and realized it'd be nicer to require that the client sends `X-Accept-Authenticate: LN-invoice header` (could be standardized to not require `X-` in the future) This allows the server to know upfront that the client actually supports LN.

Advantages:

* The server can make conditional response and e.g. send CAPTCHA instead (if it's used as rate-limiting feature) or provide on-chain address for clients who wish to avoid LN and pay bigger amount upfront. (Or use fiat :vomiting_face: )
* The server can avoid generating and storing the invoice if the endpoint was visited by accident. This is NOT meant as a protection against malicious actors attempting to DoS, just a small optimization/helper. Since various (spam, indexing...) bots today do not understand LN, it actually does serve as a spam protection **temporarily**.

Disadvantages:

* Slightly more complicated implementation
* Privacy implications if **browsers** unconditionally send the header while being minority. This can be mitigated by the server sending a header like `X-Supports-LN: example.com/*, *.example.com/*`, the browser could then offer the user to enable LN for that website.

No files, tests, or implementation entry points are named. Start by locating the HTTP authentication and payment-handling paths, then clarify whether the project should require the proposed X-Accept-Authenticate header and define compatible client and server behavior before implementation.

Written by the indexing model from the issue text.