Enforce proposer settings schema from file/URL loading

[syjn99]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

• docs: add v2 proposer settings reference for the gloas fork prysm-documentation#1242

As we have introduced new version for proposer settings schema, I can think of a scenario that an user modifies the settings file incorrectly before Gloas fork but Prysm usually ignores the file and silently takes the correct data. I think it's a better UX to say loudly when there is a undesired field. Also this can catch some common typo like fee_recipent.

This PR looks huge, but most of the diff contains new JSON/YAML files for the unit test. There are few changes in production path, namely:

• Either file/URL path, Prysm now decodes strictly (both for JSON and YAML path)
• When the version is larger than 2 or builder_set is explictly set, Prysm rejects it at startup.

Which issue(s) does this PR fix?

N/A

Other notes for review

• Add GET/POST/DELETE /eth/v1/validator/{pubkey}/builders keymanager endpoints #17261

The PR above changes BuilderConfig.relays as reserved fields. Now, this PR rollbacks with an explicit comment, as we will still allow relays field even after v2, even though it is completely ignored.

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description with sufficient context for reviewers to understand this PR.
• I have tested that my changes work as expected and I added a testing plan to the PR description (if applicable).