Publish subscriptions to all beacon nodes

[pawanjay176]

Issue Addressed

Resolves #3516

Proposed Changes

Adds a beacon fallback function for running a beacon node http query on all available fallbacks instead of returning on a first successful result. Uses the new run_on_all method for attestation and sync committee subscriptions.

Additional Info

Please provide any additional information. For example, future considerations or information useful for reviewers.

[torfbolt]

Also resolves #3422

[realbigsean]

bors r+

[bors[bot]]

Pull request successfully merged into unstable.

Build succeeded:

• arbitrary-check
• beacon-chain-tests
• cargo-audit
• cargo-fmt
• cargo-udeps
• cargo-vendor
• check-benchmarks
• check-consensus
• check-msrv
• clippy
• debug-tests-ubuntu
• dockerfile-ubuntu
• doppelganger-protection-test
• ef-tests-ubuntu
• eth1-simulator-ubuntu
• execution-engine-integration-ubuntu
• merge-transition-ubuntu
• no-eth1-simulator-ubuntu
• op-pool-tests
• release-tests-ubuntu
• release-tests-windows
• state-transition-vectors-ubuntu