go/worker/registration: Verify entity exists before node registration

[kostko]

Fixes #3286

This avoids some cases of failed node registration transactions when the entity
under which the node is being registered does not actually exist.

[codecov]

Codecov Report

Merging #3288 into master will increase coverage by 0.03%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #3288      +/-   ##
==========================================
+ Coverage   65.36%   65.39%   +0.03%     
==========================================
  Files         371      371              
  Lines       32986    33008      +22     
==========================================
+ Hits        21560    21587      +27     
+ Misses       8254     8242      -12     
- Partials     3172     3179       +7     

Impacted Files	Coverage Δ
go/worker/registration/worker.go	67.61% <75.00%> (+0.02%)	⬆️
go/consensus/tendermint/apps/staking/auth.go	50.00% <0.00%> (-25.00%)	⬇️
go/consensus/tendermint/apps/beacon/beacon.go	69.49% <0.00%> (-8.48%)	⬇️
go/consensus/tendermint/apps/beacon/state/state.go	53.33% <0.00%> (-6.67%)	⬇️
go/consensus/api/submission.go	60.31% <0.00%> (-6.35%)	⬇️
...nsensus/tendermint/apps/staking/signing_rewards.go	20.00% <0.00%> (-5.72%)	⬇️
.../consensus/tendermint/apps/epochtime_mock/state.go	75.00% <0.00%> (-5.00%)	⬇️
go/consensus/tendermint/apps/staking/query.go	40.47% <0.00%> (-4.77%)	⬇️
go/storage/metrics.go	70.76% <0.00%> (-4.62%)	⬇️
go/storage/api/root_cache.go	75.00% <0.00%> (-3.34%)	⬇️
... and 31 more

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