refactor: use echo webserver framework

[samlaf]

This is just a suggestion. Not committed to merging this if others oppose.

Current webserver with custom hand-written middlewares has some issues like:

• inconsistent logging with duplicates
• harder to extend with some useful features that come for free in webserver frameworks (eg: adding request-ids to every request for easier tracing when debugging failed requests)

TODO:

• echo enforces that handlers only return an error, and no other values, so had to leave out the commitment metadata that was used for metrics middleware. This will be fixed once fix: custom error wrapped around error and commitment meta #134 is merged and rebase on top though
• we now have 2 metrics endpoints... one for our metrics and one for echo's metrics. there's def a way to merge them but need to think about this one
• add the request-id middleware