Add support for SLH-DSA

[Jakuje]

Description

Fixes: #244.

Checklist

• Test suite updated with functionality tests
• [~] Test suite updated with negative tests
• Rustdoc string were added or updated
• CHANGELOG and/or other documentation added or updated
• [~] This is not a code change

Reviewer's checklist:

• Any issues marked for closing are fully addressed
• There is a test suite reasonably covering new functionality or modifications
• This feature/change has adequate documentation added
• A changelog entry is added if the change is significant
• Code conform to coding style that today cannot yet be enforced via the check style test
• Commits have short titles and sensible text
• Doc string are properly updated

[Jakuje]

The ugly part of SLH-DSA is the signature size. This makes the test vector 36MB(!) and quite slow (therefore marked with slow feature for now).

For the release, I would probably consider stripping these tests somehow from the cargo tarballs to prevent having them needlessly large.

[simo5]

Overall looks good, but I think we need to change the SHA3 OIDs fix

[simo5]

LGTM