Add GetEvents to backends

[abukosek]

Fixes #2778.

[kostko]

Also update the txsource query workload (see go/oasis-node/cmd/debug/txsource/workload/queries.go) to include these new methods.

[codecov]

Codecov Report

Merging #2841 into master will increase coverage by 0.16%.
The diff coverage is 67.13%.

@@            Coverage Diff             @@
##           master    #2841      +/-   ##
==========================================
+ Coverage   67.31%   67.48%   +0.16%     
==========================================
  Files         344      344              
  Lines       33089    33388     +299     
==========================================
+ Hits        22275    22532     +257     
- Misses       7935     7962      +27     
- Partials     2879     2894      +15     

Impacted Files	Coverage Δ
go/registry/api/api.go	37.57% <ø> (ø)
go/roothash/api/api.go	75.00% <ø> (ø)
go/staking/api/api.go	71.42% <ø> (ø)
go/registry/api/grpc.go	32.72% <17.64%> (-0.83%)	⬇️
go/consensus/tendermint/roothash/roothash.go	65.54% <33.33%> (-0.01%)	⬇️
go/consensus/tendermint/registry/registry.go	66.92% <53.40%> (-5.90%)	⬇️
go/consensus/tendermint/staking/staking.go	62.85% <56.00%> (-2.77%)	⬇️
go/staking/api/grpc.go	52.61% <64.70%> (-0.33%)	⬇️
go/consensus/tendermint/tendermint.go	68.10% <77.77%> (+1.25%)	⬆️
go/registry/tests/tester.go	91.40% <100.00%> (+0.78%)	⬆️
... and 32 more

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[abukosek]

Thanks for the review, I've addressed all the comments, so please re-review :)

[kostko]

Aw you forgot to squash :/