Convert RpcBlock to an enum that indicates availability #24917

Job	Run time
lockbud	41s
target-branch-check	2s
check-code	12m 12s
check-msrv	4m 48s
Check for 'skip-ci' label	3s
http-api-tests	21m 50s
beacon-chain-tests	14m 27s
slasher-tests	3m 2s
release-tests-ubuntu	16m 29s
network-tests	16m 15s
state-transition-vectors-ubuntu	6m 45s
op-pool-tests	11m 17s
execution-engine-integration-ubuntu	7m 51s
compile-with-beta-compiler	14m 2s
fallback-simulator-ubuntu	18m 0s
basic-simulator-ubuntu	20m 10s
cargo-udeps	7m 4s
cargo-sort	9s
debug-tests-ubuntu	5m 30s
ef-tests-ubuntu	8m 24s
cli-check	13m 52s
test-suite-success	8s
	3h 23m 1s

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