fabricx: use idemix with new curves to sign transactions

[Rozerxshashank]

Closes : #1093

Description

This PR addresses the hardcoded FP256BN_AMCL curve in the Idemix implementation. The existing logic made it difficult to adopt newer, more efficient elliptic curves like BN254 (via mathlib/gnark-crypto), which offer better performance and avoid known race conditions in the older AMCL library.
I've refactored the identity loading pipeline to make the elliptic curve configurable while maintaining strict backward compatibility.

Changes

• Config: Added an optional curveID field to the MSP configuration struct.
• Loader: Updated the identity loader to parse the curveID string and pass the corresponding math.CurveID to the providers.
• Deserializer: Added a curve-configurable constructor for the deserializer to ensure consistent verification of remote identities.
• Membership Interface: Extended LocalMembership with RegisterIdemixMSPWithCurve to allow programmatic curve selection.
• Backward Compatibility: All existing methods (RegisterIdemixMSP, NewProviderWithAnyPolicy, NewDeserializer) now delegate to the new curve-aware variants using the legacy FP256BN_AMCL as the default.

Testing

Verified the changes with the following steps:

• Ran all Idemix provider unit tests (platform/fabric/core/generic/msp/idemix): Passed.
• Ran full platform build: Succeeded.
• Verified linting with golangci-lint run --fix: 0 issues.

Manual Configuration

You can now specify the curve in your network configuration:

fabric:
  msps:
    - id: idemix
      mspType: idemix
      mspID: IdemixOrgMSP
      path: ./crypto/idemix
      curveID: BN254  # Supported: BN254, FP256BN_AMCL, BLS12_381_BBS, etc.

[Rozerxshashank]

HI @adecaro, please review this when you get time. Thanks ☺️