docs: fix getCreator and ProposalCreator type documentation

[JhaSourav07]

Description

This PR fixes the JSDoc documentation for the getCreator() method and the submitter's identity object to accurately reflect the TypeScript definitions (SerializedIdentity) found in fabric-shim-api.

Previously, the JSDoc incorrectly referred to the identity object as an instantiable class named ProposalCreator and incorrectly listed the certificate bytes property as id_bytes instead of the camelCased idBytes.

Changes Made:

• Renamed ProposalCreator to SerializedIdentity in libraries/fabric-shim/lib/stub.js.
• Updated the certificate bytes property name from id_bytes to idBytes.
• Removed the @class tag from the identity object typedef so it is no longer falsely documented as a constructible class.
• Updated the getCreator() return type and other relevant references (like SignedProposal and SignatureHeader) to use SerializedIdentity.

Related Issues

Fixes #308

Checklist:

• I have added a Developer Certificate of Origin (DCO) sign-off to my commit(s).
• I have run the linter and unit tests locally to verify my changes do not break existing code.
• I have built the documentation locally to verify the changes display correctly.

[bestbeforetoday]

Thank you for the contribution.