Remove or implement dead CreateOptions.signers field

[eugenioenko]

Description

CreateOptions.signers is part of the public OpenTDF.createTDF() API but is never read or forwarded. Supplying it has no runtime effect.

Assertion signing currently uses assertionConfigs[].signingKey. DPoP signing uses dpopKeys. The vendored JOSE helpers do not consume CreateOptions.signers.

Expected outcome

Choose one:

• Remove CreateOptions.signers as dead API, with the appropriate compatibility plan.
• Implement and document how signers maps into the TDF creation and assertion-signing flow.

Add coverage proving the selected behavior.