refactor: Make validator_signer mutable #5161

Job	Run time
Protobuf Backward Compatibility	9s
Cargo Nextest (Linux)	16m 28s
Cargo Clippy	8m 20s
Cargo Nextest (Linux StatelessNet)	17m 21s
Cargo Fmt	18s
Rust Doctests	11m 7s
Cargo Nextest (Linux Nightly)	16m 42s
Cargo Nextest (MacOS)	19m 4s
Backward Compatibility	10m 42s
Lychee Lints	15s
Database Migration	12m 41s
Genesis Changes	9m 27s
Sanity Checks	6m 4s
Style	37s
Upgradability	10m 51s
RPC Schema	11m 30s
Cargo Audit	38s
Cargo Deny	29s
Non-default Configuration Builds	1m 18s
Themis	38s
Unused Dependencies	6m 45s
Windows check for building public libraries	5m 25s
Upload Coverage	24s
Generate Coverage Artifact (unit, unit)	2m 56s
Generate Coverage Artifact (integration, unit integration)	3m 59s
	2h 54m 8s

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