feat: add block height to `commit-block` event

[obycode]

The event generated by a commit-block (on the L1) now includes the block height for the block that is being committed, in the block-height key of the event tuple. The target burnchain block height now uses the key target-burn-block-height. This is useful for debugging, making it easy to match the L1 transactions with the L2 blocks. Note that this field is not checked in the tests since it is purely for convenience and could be left out of some implementations.

[codecov[bot]]

Codecov Report

Merging #260 (5002792) into master (7d373f0) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #260      +/-   ##
==========================================
+ Coverage   93.47%   93.49%   +0.01%     
==========================================
  Files           6        6              
  Lines         337      338       +1     
==========================================
+ Hits          315      316       +1     
  Misses         22       22              

Impacted Files	Coverage Δ
core-contracts/contracts/multi-miner.clar	97.05% <100.00%> (ø)
core-contracts/contracts/subnet.clar	96.52% <100.00%> (+0.01%)	:arrow_up:

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[obycode]

The last force push fixed the failing test. I'll merge once the tests all pass with this latest version.