Feat/signers subscribe to burn blocks

[jferrant]

• Subscribes to burn block events
• Store current reward cycle on the side. Only refresh reward cycle when a new burn block event arrives (prevents spamming the node needlessly)
• Only attempt to initialize/refresh the current reward cycle signer if initializing the runloop
• Only attempt to refresh a signer config for the NEXT reward cycle if in the prepare phase during runloop initialization OR have received a new burn block in the prepare phase and have still not been registered
• Removes unused ClientErrors
• This uncovered a bug in poststackerdbchunk error codes. Cleaned up as part of this PR

Closes: #4504

[codecov]

Codecov Report

Attention: Patch coverage is 85.05747% with 39 lines in your changes are missing coverage. Please review.

Project coverage is 77.52%. Comparing base (6278946) to head (48bbbd5).
Report is 22 commits behind head on next.

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #4549      +/-   ##
==========================================
- Coverage   83.35%   77.52%   -5.83%     
==========================================
  Files         453      453              
  Lines      328223   328494     +271     
  Branches      323      323              
==========================================
- Hits       273578   254676   -18902     
- Misses      54637    73810   +19173     
  Partials        8        8              

Files	Coverage Δ
stacks-signer/src/client/mod.rs	74.07% <ø> (-25.08%)	⬇️
stacks-signer/src/client/stackerdb.rs	89.79% <0.00%> (ø)
stackslib/src/net/api/poststackerdbchunk.rs	81.11% <87.50%> (-1.05%)	⬇️
stacks-signer/src/client/stacks_client.rs	75.31% <78.57%> (-11.95%)	⬇️
testnet/stacks-node/src/tests/signer.rs	94.34% <91.30%> (+0.47%)	⬆️
stacks-signer/src/signer.rs	75.02% <65.21%> (-0.40%)	⬇️
libsigner/src/events.rs	77.69% <62.50%> (-1.90%)	⬇️
stacks-signer/src/runloop.rs	91.50% <90.98%> (+0.76%)	⬆️

... and 151 files with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6278946...48bbbd5. Read the comment docs.

[jcnelson]

LGTM; just minor nits

[xoloki]

LGTM just a tiny question about caching the approved agg key at various layers