go/staking: Add DebondingStartEscrowEvent

[kostko]

This makes it easier to reason about the debonded amounts, both in shares and
in base units.

[codecov]

Codecov Report

Merging #4014 (f31bc77) into master (5248989) will decrease coverage by 0.06%.
The diff coverage is 77.77%.

❗ Current head f31bc77 differs from pull request most recent head 2c2a01b. Consider uploading reports for the commit 2c2a01b to get more accurate results

@@            Coverage Diff             @@
##           master    #4014      +/-   ##
==========================================
- Coverage   67.02%   66.96%   -0.07%     
==========================================
  Files         412      412              
  Lines       42263    42298      +35     
==========================================
- Hits        28328    28325       -3     
- Misses       9945     9972      +27     
- Partials     3990     4001      +11     

Impacted Files	Coverage Δ
go/staking/api/api.go	58.87% <ø> (ø)
go/consensus/tendermint/staking/staking.go	55.72% <57.14%> (+0.05%)	⬆️
go/staking/tests/tester.go	91.49% <80.00%> (-0.33%)	⬇️
.../consensus/tendermint/apps/staking/transactions.go	68.54% <88.88%> (+0.69%)	⬆️
...nsensus/tendermint/apps/beacon/state/state_pvss.go	69.23% <0.00%> (-7.70%)	⬇️
go/consensus/tendermint/apps/beacon/genesis.go	62.50% <0.00%> (-6.25%)	⬇️
go/worker/keymanager/handler.go	69.38% <0.00%> (-6.13%)	⬇️
...nsensus/tendermint/apps/staking/signing_rewards.go	20.00% <0.00%> (-5.72%)	⬇️
go/runtime/nodes/grpc/grpc.go	71.93% <0.00%> (-3.56%)	⬇️
go/keymanager/client/client.go	76.31% <0.00%> (-3.51%)	⬇️
... and 17 more

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[Yawning]

Seems fine to me.