go/oasis-node/cmd/common: Add LoadEntitySigner() helper

[tjanez]

Replace LoadEntity() helper with LoadEntitySigner() that requires no arguments and loads the entity directory and obtains the signer backend by itself.

Remove entity directory loading boilerplate code by using this new helper.

[codecov]

Codecov Report

Merging #3556 (84428ba) into master (2c3a473) will decrease coverage by 0.02%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master    #3556      +/-   ##
==========================================
- Coverage   66.52%   66.50%   -0.03%     
==========================================
  Files         377      377              
  Lines       34296    34273      -23     
==========================================
- Hits        22815    22792      -23     
  Misses       8189     8189              
  Partials     3292     3292              

Impacted Files	Coverage Δ
go/oasis-node/cmd/common/common.go	49.60% <40.00%> (-0.40%)	⬇️
go/oasis-node/cmd/registry/entity/entity.go	42.92% <50.00%> (+0.67%)	⬆️
go/oasis-node/cmd/registry/runtime/runtime.go	57.69% <50.00%> (+0.77%)	⬆️
go/oasis-node/cmd/registry/node/node.go	44.35% <66.66%> (+0.44%)	⬆️
go/oasis-node/cmd/common/consensus/consensus.go	42.16% <100.00%> (+1.05%)	⬆️
go/staking/api/grpc.go	52.75% <0.00%> (-6.30%)	⬇️
go/consensus/tendermint/apps/registry/registry.go	57.73% <0.00%> (-5.16%)	⬇️
go/worker/common/committee/runtime_host.go	65.04% <0.00%> (-4.86%)	⬇️
go/consensus/tendermint/apps/staking/query.go	35.71% <0.00%> (-4.77%)	⬇️
go/runtime/tagindexer/tagindexer.go	66.27% <0.00%> (-4.66%)	⬇️
... and 26 more

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