[WIP] External API support for extmu

[abhi-dev-engg]

Add External API support for "External MU" Signature generation and verification

[abhi-dev-engg]

Hi @bhess ,

I was trying to implement the external mu OQS APIs based on approach 1 suggested here by you. However, in my (limited) understanding it seems difficult adding a "new variant" without having support from the upstream repo(see one error below).

Tried adding the new variants "ml_dsa_xx_extmu" in 'copy_from_upstream.yml' under ml-dsa umbrella leading to 'FileNotFoundError: [Errno 2] No such file or directory: 'repos/mldsa-native/integration/liboqs/ML-DSA-44-EXTMU_META.yml'.

Additionally, the extmu and the existing variants shall be using the same set of source files, which could lead to linking errors due to "same definition" if both the variant are enabled.

Please let me know your opinion or please guide me incase I am in the wrong direction.

Thanks.