keymanager: Rename APIs referencing "contracts"

[Yawning]

Each runtime does not neccecarily have a notion for contracts, so the
key manager now operates in terms of KeyPairIds that identify a given
KeyPair.

Fixes #2844.

[codecov]

Codecov Report

Merging #2979 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2979      +/-   ##
==========================================
- Coverage   68.57%   68.52%   -0.05%     
==========================================
  Files         360      360              
  Lines       35273    35273              
==========================================
- Hits        24188    24171      -17     
- Misses       7988     8000      +12     
- Partials     3097     3102       +5     

Impacted Files	Coverage Δ
go/worker/compute/executor/committee/state.go	74.07% <0.00%> (-11.12%)	⬇️
go/storage/metrics.go	84.78% <0.00%> (-6.53%)	⬇️
go/consensus/tendermint/apps/registry/query.go	80.95% <0.00%> (-4.77%)	⬇️
go/storage/api/root_cache.go	72.41% <0.00%> (-4.60%)	⬇️
.../consensus/tendermint/apps/epochtime_mock/state.go	78.72% <0.00%> (-4.26%)	⬇️
go/storage/mkvs/insert.go	87.75% <0.00%> (-4.09%)	⬇️
go/storage/mkvs/tree.go	89.28% <0.00%> (-3.58%)	⬇️
go/storage/database/database.go	72.26% <0.00%> (-3.37%)	⬇️
go/worker/compute/executor/committee/node.go	61.03% <0.00%> (-3.27%)	⬇️
go/consensus/tendermint/registry/registry.go	68.89% <0.00%> (-3.15%)	⬇️
... and 27 more

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