json decoding: allow unknown fields (in preparation for Capella and EIP-4844)

[metachris]

📝 Summary

This PR allows unknown fields in the JSON request payloads.

• New payload fields will be part of the upcoming Capella and EIP-4844 updates (see also Changes for Capella and EIP-4844 #392)
• These new payload fields would make mev-boost break on the getPayload call unless a new API version is introduced
• Since there's multiple upcoming changes, it seems to be overkill to add a new API version for each update
• The simple solution is to allow unknown fields in the json payload, and have all users upgrade before any payload changes are deployed. This way the can be backwards compatible.
• This limitation was applied only to the registerValidator and getPayload request payload from the beacon-node, where there is not real risk of resource exhaustion (and in case of getPayload the payload was fully read beforehand anyway)

I think this should go into the v1.4.0 release (see #377) in order to strengthen backwards compatibility with future payload additions.

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✅ I have run these commands

• make lint
• make test-race
• go mod tidy

[codecov-commenter]

Codecov Report

Merging #393 (e9775da) into main (f146349) will increase coverage by 0.14%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main     #393      +/-   ##
==========================================
+ Coverage   66.07%   66.21%   +0.14%     
==========================================
  Files           7        7              
  Lines         902      894       -8     
==========================================
- Hits          596      592       -4     
+ Misses        271      267       -4     
  Partials       35       35              

Flag	Coverage Δ
unittests	66.21% <33.33%> (+0.14%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/service.go	78.57% <0.00%> (+0.92%)	⬆️
server/utils.go	72.72% <ø> (-2.95%)	⬇️
server/mock_relay.go	86.92% <100.00%> (ø)

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