Context support for ML-DSA and SLH-DSA

[FAlbertDev]

ML-DSA and SLH-DSA allow for a custom context for domain separation. This PR adds support for this using the new (experimental) signature options interface.

This closes #4376 since pre-hashed variants are postponed for now (see https://github.com/randombit/botan/issues/4376#issuecomment-2598613934).

Dependencies

• #4318

TODOs

• [ ] Tests

[coveralls]

coverage: 91.206% (-0.003%) from 91.209% when pulling 34786d9debd4b1f606d89c6089a9f8a707b6cbd5 on Rohde-Schwarz:feature/fips_ctx_and_prehash into 6a97b80e934bb3903d4ce363db0adadc06e22270 on randombit:master.