keymanager: Refactor and reorganize runtime-related crates

[peternose]

Currently there is a bunch of runtime-related crates, where the split is mostly for legacy reasons. We should consider how to consolidate these crates so that it makes them easier to maintain. The current crates are as follows:

• oasis-core-client (client/), could be moved into oasis-core-runtime.
• oasis-core-keymanager-* (keymanager-*/), some (APIs?) could be moved into oasis-core-runtime, some (KM implementation) could be consolidated under a single crate.

[codecov]

Codecov Report

Merging #4930 (98bc4da) into master (06012a8) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4930      +/-   ##
==========================================
- Coverage   66.64%   66.62%   -0.03%     
==========================================
  Files         464      464              
  Lines       51204    51204              
==========================================
- Hits        34127    34113      -14     
- Misses      12891    12901      +10     
- Partials     4186     4190       +4     

Impacted Files	Coverage Δ
go/common/version/version.go	80.26% <ø> (ø)
...onsensus/tendermint/apps/beacon/state/state_vrf.go	73.33% <0.00%> (-13.34%)	⬇️
.../worker/compute/executor/committee/transactions.go	79.54% <0.00%> (-9.10%)	⬇️
go/worker/beacon/tx_retry.go	90.47% <0.00%> (-4.77%)	⬇️
...consensus/tendermint/apps/roothash/transactions.go	66.66% <0.00%> (-3.12%)	⬇️
go/worker/common/p2p/rpc/client.go	80.93% <0.00%> (-2.80%)	⬇️
go/oasis-node/cmd/ias/auth_registry.go	65.81% <0.00%> (-2.57%)	⬇️
go/runtime/txpool/txpool.go	82.70% <0.00%> (-2.44%)	⬇️
...nsensus/tendermint/apps/beacon/backend_insecure.go	58.24% <0.00%> (-2.20%)	⬇️
go/consensus/tendermint/apps/beacon/state/state.go	68.81% <0.00%> (-2.16%)	⬇️
... and 22 more

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[kostko]

Nice work!