Implement `RegisterSubnetValidatorTx` #7344

Job	Run time
Fuzz	12m 41s
Unit (macos-12)	8m 48s
Unit (ubuntu-20.04)	5m 52s
Unit (ubuntu-22.04)	5m 50s
e2e_post_etna	1m 37s
Unit (windows-2022)	9m 28s
e2e_pre_etna	1m 36s
Unit (custom-arm64-focal)	10m 54s
Unit (custom-arm64-jammy)	10m 15s
Protobuf Lint	4s
e2e_existing_network	1m 43s
Lint	3m 2s
Up-to-date protobuf	33s
Upgrade	1m 24s
Up-to-date mocks	1m 46s
Up-to-date go.mod and go.sum	13s
Image build	1m 57s
Build Antithesis avalanchego images	1m 36s
Build Antithesis xsvm images	1m 40s
	1h 20m 59s

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