Support signing BlobSidecar with Web3Signer

[jimmygchen]

Description

Support Web3Signer signing for blob sidecars. Depends on this Web3Signer issue: Consensys/web3signer#726

Once the schema and signing is implemented in Web3Signer, the VC needs to construct a Web3SignerObject here:

lighthouse/validator_client/src/signing_method.rs

Lines 187 to 190 in 4c9fcf1

	SignableMessage::BlobSidecar(_) => {
	// https://github.com/ConsenSys/web3signer/issues/726
	unimplemented!("Web3Signer blob signing not implemented.")
	}

[jimmygchen]

Remote signing api spec PR merged here: ethereum/remote-signing-api#4

TL;DR:

For blob sidecar signing we can use the blinded sidecar type for both using the blob_root instead of the full blob.

[realbigsean]

Closing due to deprecation of blob signing