Evaluate behavior when multiple signatures in a Rollkit Commit

[Manav-Aggarwal]

Rollkit currently does not support a multi-signature scheme so there can only be one signature in a header commit. Currently, while computing a commitHash, Rollkit only supports and checks if the number of signatures in the Commit is 1. We should consider throwing an error in this case.

And what happens if multiple signatures are present? Shouldn't this be an error?

Originally posted by @tzdybal in #1026 (comment)