feat: bls signer abstraction

[shrimalmadhur]

Fixes # .

What Changed?

• Introducing a separate module for the signer. We are thinking of making eigensdk a multi module for better version management and usage experience.
• Re-using the signer folder (which is deprecated) for new signer abstraction

Reviewer Checklist

• Code is well-documented
• Code adheres to Go naming conventions
• Code deprecates any old functionality before removing it

[ian-shim]

lgtm, can we add some tests?

[shrimalmadhur]

lgtm, can we add some tests?

cool ya - I will add those tests and then I will also integrate with eigenda