SQIsign integration

[bhess]

PR builds on top of #2174, authored by @shane-digi.

Fixes #1946

Integrates the SQIsign signature family copied from upstream and makes GMP an optional, dynamically-detected dependency.

• Add sqisign implementations (copied from upstream), without patches.
• Since SQIsign depends on the GMP library, it is only enabled if GMP is available.
• Makes GMP dependency optional. OQS_USE_GMP is enabled by default if a system installation of GMP is found. The dependency can also be explicitly disabled with -DOQS_USE_GMP=OFF.

Marking this as draft until #2174 (comment) has been resolved.

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged. Also, make sure to update the list of algorithms in the continuous benchmarking files: .github/workflows/kem-bench.yml and sig-bench.yml)