core: deprecate VersionedSignedBlindedProposal

[gsora]

Convert VersionedSignedProposals to their blinded counterpart (requested by go-eth2-client & Beacon API spec) on-demand, so that the execution flow is simpler to understand and debug.

category: refactor ticket: none

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 16.66667% with 50 lines in your changes missing coverage. Please review.

Project coverage is 56.72%. Comparing base (551c898) to head (e76df29).

Files	Patch %	Lines
core/signeddata.go	0.00%	37 Missing :warning:
core/bcast/bcast.go	33.33%	8 Missing :warning:
core/tracker/inclusion.go	54.54%	3 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3155 +/- ## ========================================== - Coverage 56.89% 56.72% -0.18% ========================================== Files 208 208 Lines 29365 29202 -163 ========================================== - Hits 16707 16564 -143 + Misses 10817 10811 -6 + Partials 1841 1827 -14 ```

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[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud