[bug]: schnorr signature does not allow messages with length different than 32 bytes

[allocz]

The current implementation fails the bip340 vector tests because the signature verification fails when the message length is not 32 bytes. Both bip340 test vectors and libsecp256k1 allows messages with variable length.

The fix for this bug is to remove the restrictions of the message length in both schnorr signing and verifying procedures.

I can provide the PR with the fix and tests upon ACK from maintainers.

[allocz]

This also closes #2501, I could not find an issue linked to it.