runtime/storage/mkvs: Add MKVS impl for &mut T

[kostko]

Trivial MKVS trait impl needed by the ParaTime SDK.

[codecov]

Codecov Report

Merging #3604 (e87d79e) into master (205e569) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3604      +/-   ##
==========================================
- Coverage   67.25%   67.17%   -0.08%     
==========================================
  Files         390      390              
  Lines       36507    36507              
==========================================
- Hits        24552    24523      -29     
- Misses       8498     8522      +24     
- Partials     3457     3462       +5     

Impacted Files	Coverage Δ
go/consensus/tendermint/apps/governance/query.go	81.81% <0.00%> (-18.19%)	⬇️
go/consensus/tendermint/full/services.go	78.81% <0.00%> (-6.78%)	⬇️
go/common/sgx/aesm/aesm.go	62.61% <0.00%> (-6.55%)	⬇️
go/storage/mkvs/insert.go	87.67% <0.00%> (-5.48%)	⬇️
go/runtime/nodes/grpc/grpc.go	71.08% <0.00%> (-5.23%)	⬇️
go/worker/upgrade/worker.go	67.70% <0.00%> (-5.21%)	⬇️
go/keymanager/client/client.go	77.19% <0.00%> (-4.39%)	⬇️
go/storage/mkvs/tree.go	86.95% <0.00%> (-4.35%)	⬇️
go/storage/api/root_cache.go	78.33% <0.00%> (-3.34%)	⬇️
.../consensus/tendermint/apps/roothash/state/state.go	74.28% <0.00%> (-2.86%)	⬇️
... and 31 more

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