go/oasis-node: Omit existing entity check for non-file signers

[kostko]

Fixes #3215

The "registry entity init" subcommand previously always performed a check
whether an entity already exists. It did that by creating an additional signer
factory to perform this check.

Some signers assign exclusive access to an underlying resource (e.g., HSM) to
the given factory. In that case, all operations on the second signer factory
would fail. Thus we now omit the existing entity check for non-file signers.

[codecov]

Codecov Report

Merging #3216 into master will increase coverage by 0.39%.
The diff coverage is 18.18%.

@@            Coverage Diff             @@
##           master    #3216      +/-   ##
==========================================
+ Coverage   67.38%   67.78%   +0.39%     
==========================================
  Files         370      370              
  Lines       36098    36101       +3     
==========================================
+ Hits        24326    24470     +144     
+ Misses       8628     8499     -129     
+ Partials     3144     3132      -12     

Impacted Files	Coverage Δ
go/oasis-node/cmd/registry/entity/entity.go	45.96% <18.18%> (+0.25%)	⬆️
go/consensus/tendermint/api/errors.go	86.66% <0.00%> (-13.34%)	⬇️
go/runtime/host/sandbox/sandbox.go	67.28% <0.00%> (-10.04%)	⬇️
go/runtime/host/protocol/connection.go	62.16% <0.00%> (-4.25%)	⬇️
go/consensus/tendermint/apps/staking/state/gas.go	81.03% <0.00%> (-3.45%)	⬇️
go/storage/mkvs/checkpoint/checkpointer.go	66.17% <0.00%> (-1.48%)	⬇️
go/consensus/tendermint/roothash/roothash.go	72.31% <0.00%> (-0.85%)	⬇️
go/storage/api/root_cache.go	77.77% <0.00%> (ø)
go/worker/common/committee/node.go	75.70% <0.00%> (ø)
...o/consensus/tendermint/apps/staking/state/state.go	59.29% <0.00%> (ø)
... and 25 more

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