ml-dsa: add `ComputeMu()`

[daxpedda]

In #1023 we added methods for SignMu() and VerifyMu(), which basically allows for pre-hashing. However in practice, to compute Mu, you have to hash the public key tr, even though it is already present in both SigningKey::tr and VerifyingKey::tr.

Therefor I propose adding SigningKey::compute_mu() and VerifyingKey::compute_mu() to facilitate external mu signing and verifying.

[daxpedda]

Just for future context how this was resolves:

RustCrypto/traits#2003 allows DigestSigner/Verifier to work for ML-DSA as well, by providing a pre-hashed Digest to the user. This was done in #1064.

However, we still need VerifyingKey::compute_mu() to support HSM use-cases. While this hasn't made it into PKCS#11 yet (oasis-tcs/pkcs11#19), e.g. AWS supports this already. This is done in #1074.