Update opamp-spec to v0.12.0

[michel-laterman]

Update the opamp-spec to https://github.com/open-telemetry/opamp-spec/releases/tag/v0.12.0 And regenerate the ProtoBufs.
Required to unblock #341

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.57%. Comparing base (182c2bb) to head (94a2508).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #362      +/-   ##
==========================================
- Coverage   80.10%   79.57%   -0.54%     
==========================================
  Files          25       25              
  Lines        2423     2423              
==========================================
- Hits         1941     1928      -13     
- Misses        374      384      +10     
- Partials      108      111       +3     

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[michel-laterman]

@tigrannajaryan, @andykellr, can I please get a review for this?

[tigrannajaryan]

Let's make a opamp-spec v0.12.0 release and consume that instead of consuming a specific unreleased submodule commit.

I will prepare a changelog: open-telemetry/opamp-spec#215

[tigrannajaryan]

opamp-spec v0.12.0 released, please use it in this PR: https://github.com/open-telemetry/opamp-spec/releases/tag/v0.12.0