go/keymanager/api: Add signed encrypted master secret

[peternose]

Master secret rotation, 2nd part:

• Add signed encrypted master secret.

[codecov]

Codecov Report

Merging #5199 (a59439d) into master (cf5e508) will decrease coverage by 0.22%.
The diff coverage is 40.94%.

@@            Coverage Diff             @@
##           master    #5199      +/-   ##
==========================================
- Coverage   61.47%   61.25%   -0.22%     
==========================================
  Files         512      512              
  Lines       54051    54230     +179     
==========================================
- Hits        33227    33218       -9     
- Misses      16611    16786     +175     
- Partials     4213     4226      +13     

Impacted Files	Coverage Δ
go/registry/api/api.go	52.78% <0.00%> (-0.16%)	⬇️
go/worker/compute/executor/committee/node.go	58.90% <22.22%> (-0.27%)	⬇️
go/runtime/host/multi/multi.go	45.52% <24.80%> (-21.38%)	⬇️
go/worker/common/committee/node.go	57.36% <40.00%> (-0.58%)	⬇️
go/runtime/host/sandbox/sandbox.go	56.65% <50.00%> (-0.20%)	⬇️
go/worker/keymanager/worker.go	63.36% <50.00%> (+1.98%)	⬆️
go/runtime/registry/host.go	49.90% <66.66%> (+0.20%)	⬆️
go/keymanager/api/secret.go	76.31% <78.57%> (+18.42%)	⬆️
...nsensus/tendermint/apps/keymanager/transactions.go	42.53% <100.00%> (ø)
go/registry/api/runtime.go	51.42% <100.00%> (+1.01%)	⬆️
... and 33 more

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