feat: add pox_stx_threshold amount to /new_block event data

[zone117x]

Event indexers (e.g. the Stacks API) analyze historical pox-4 events to generate rich signer data. One of the challenges is determining which past stacking transactions should be considered for a given pox cycle. In this case, we want the ability to know if a historical pox-4 stacking event was discarded because of not meeting the minimum stx threshold (i.e. not given any slots/weight).

This PR adds the (stx) threshold value used for a given cycle's reward calculations to the /new_block event payload.

[obycode]

lgtm!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.41%. Comparing base (87c0c00) to head (d38a9e6).

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #4524      +/-   ##
==========================================
- Coverage   83.29%   82.41%   -0.88%     
==========================================
  Files         452      452              
  Lines      326071   326098      +27     
  Branches      323      323              
==========================================
- Hits       271594   268756    -2838     
- Misses      54469    57334    +2865     
  Partials        8        8              

Files	Coverage Δ
stacks-signer/src/client/stacks_client.rs	87.25% <100.00%> (+0.01%)	⬆️
stackslib/src/chainstate/coordinator/tests.rs	90.79% <100.00%> (+<0.01%)	⬆️
stackslib/src/chainstate/stacks/boot/mod.rs	94.78% <100.00%> (+0.02%)	⬆️

... and 53 files with indirect coverage changes

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[blockstack-devops]

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