Add signature utility functions

[Bez625]

Utility methods requested in this comment #182 (comment)

[KaloyanTanev]

Much appreciated!

[KaloyanTanev]

@Bez625 do you think adding AggregatorIndex() as well makes sense?
And also adding SelectionProof() to the Versioned counterpart?

[Bez625]

@Bez625 do you think adding AggregatorIndex() as well makes sense? And also adding SelectionProof() to the Versioned counterpart?

It's definitely possible to add those, but I can't see anywhere we would use them at the minute. Are they functions you would find useful? If so I'm happy to raise the change

[KaloyanTanev]

Yes, we have a couple of places throughout our Charon codebase where those will be quite handy. One example is here. I am currently refactoring the code to encompass the SubmitAggregateAttestationsOpts (and subsequently VersionedSignedAggregateAndProof) and being able to call directly agg.AggregatorIndex() as we've been doing previously with agg.Message.AggregatorIndex would be nice.