Integrate SLH-DSA-C Library

[h2parson]

This pull request integrates an implementation of SLH-DSA taken from PQCP

This PR does not resolve any existing GitHub issues.

The integrated signature algorithms can be tested using the test_sig.c file.

-This PR does not change the behaviour of existing cryptographic algorithm implementations.

-This PR will change the list of algorithms available as it adds an upstream source for slh-dsa