go/common/crypto/signature: Add and use the plugin backed signer

[Yawning]

Bloating the repository with a ton of different HSM (etc) signing backends doesn't make sense, and is a maintenance burden. Use the runtime's built-in DLSO support to be able to separate out the non-esssential implementations.

• Add the plugin signer implementation
• Change oasis-node to instantiate the plugin signer.
• Add tests for the plugin signer.

After merge:

• Move the ledger implementation off into a separate repository (Move Ledger signer plugin to this repository oasis-core-ledger#13)
• Remove the integrated ledger signer implementation.
• Remove the ledger oasis-node sub command.

Notes:

• The plugin loader enforces (rather strictly) that the package versions of plugins are the same as what the loading application was built with. This will require everything be built with -trimpath, but we should be doing that already anyway. See plugin: panic: plugin was built with a different version of package golang/go#27751 for more information.
• This is undocumented because anyone that can't figure it out, has zero business writing anything that is even vaguely security critical. That said there's tests and an example now.
• golang-ci-lint got mad when linting the plugin because it doesn't have main defined, so it is skipped for now.

[codecov]

Codecov Report

Merging #3120 into master will increase coverage by 0.34%.
The diff coverage is 13.04%.

@@            Coverage Diff             @@
##           master    #3120      +/-   ##
==========================================
+ Coverage   68.60%   68.95%   +0.34%     
==========================================
  Files         375      376       +1     
  Lines       37037    37059      +22     
==========================================
+ Hits        25409    25553     +144     
+ Misses       8389     8273     -116     
+ Partials     3239     3233       -6     

Impacted Files	Coverage Δ
...o/common/crypto/signature/signers/plugin/plugin.go	0.00% <0.00%> (ø)
go/oasis-node/cmd/common/signer/signer.go	52.68% <37.50%> (-12.43%)	⬇️
go/oasis-node/cmd/common/metrics/disk.go	65.51% <0.00%> (-20.69%)	⬇️
...n/crypto/signature/signers/ledger/ledger_signer.go	0.00% <0.00%> (-14.00%)	⬇️
go/runtime/host/sandbox/sandbox.go	67.28% <0.00%> (-10.04%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	84.00% <0.00%> (-8.00%)	⬇️
go/runtime/tagindexer/tagindexer.go	68.47% <0.00%> (-4.35%)	⬇️
.../consensus/tendermint/apps/epochtime_mock/state.go	74.46% <0.00%> (-4.26%)	⬇️
go/storage/mkvs/tree.go	89.28% <0.00%> (-3.58%)	⬇️
... and 44 more

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