Add ClientMetadata to BlobHeader

[litt3]

Description

• Add a field ClientMetadata to the BlobHeader
• This field can be used arbitrarily by the rollup, but the intention is for the rollup to encode data that can be used to obtain the correct CertVerifierAddress, to verify the blob
• One way to encode this information would be a SubmissionBlockNumber, defined as "the lastest eth block known by the batcher at the time of submission". The SBN could then be mapped to a CertVerifierAddress by the rollups internal config contracts
  • If using the SBN strategy, the rollup would also need to enforce a punctuality check of some sort, to make sure that the batcher doesn't lie and say the SBN is in the very far past

Priority

• This should be added in the near term, but not prior to v2 testnet launch