go/keymanager: Move master and ephemeral secrets code to extension

[peternose]

Having code for master and ephemeral secrets and code for churp in the same package is not transparent and introduces naming problems. This PR tries to refactor the code to achieve the following result:

// Backend is a key manager management implementation.
type Backend interface {
	Secrets() secrets.Backend

	Churp() churp.Backend
}

Separating runtime suspension (suspendRuntimes) and status generation (generateStatus) on epoch change makes this PR consensus breaking.

[codecov]

Codecov Report

Attention: 220 lines in your changes are missing coverage. Please review.

Comparison is base (191d346) 66.91% compared to head (696f204) 66.88%.
Report is 3 commits behind head on master.

❗ Current head 696f204 differs from pull request most recent head 0ac48f4. Consider uploading reports for the commit 0ac48f4 to get more accurate results

Files	Patch %	Lines
go/keymanager/secrets/grpc.go	53.33%	62 Missing and 22 partials ⚠️
...nsensus/cometbft/apps/keymanager/secrets/status.go	80.10%	26 Missing and 11 partials ⚠️
...sensus/cometbft/apps/keymanager/secrets/genesis.go	48.88%	16 Missing and 7 partials ⚠️
go/consensus/cometbft/keymanager/secrets/client.go	72.94%	16 Missing and 7 partials ⚠️
...o/consensus/cometbft/apps/keymanager/keymanager.go	65.38%	10 Missing and 8 partials ⚠️
.../consensus/cometbft/apps/keymanager/secrets/ext.go	70.58%	7 Missing and 3 partials ⚠️
.../consensus/cometbft/apps/keymanager/secrets/txs.go	64.70%	0 Missing and 6 partials ⚠️
go/keymanager/secrets/api.go	86.66%	5 Missing and 1 partial ⚠️
go/oasis-node/cmd/keymanager/keymanager.go	76.47%	3 Missing and 1 partial ⚠️
go/consensus/cometbft/apps/keymanager/genesis.go	60.00%	1 Missing and 1 partial ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5544      +/-   ##
==========================================
- Coverage   66.91%   66.88%   -0.04%     
==========================================
  Files         539      546       +7     
  Lines       56883    56976      +93     
==========================================
+ Hits        38064    38106      +42     
- Misses      14379    14420      +41     
- Partials     4440     4450      +10     

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