Re-design ssh Key as Verifier only

[lukpueh]

Previously, ssh Key satisfied both the dsse.Verifier interface and served as TUF metadata key container. Unfortunately, it didn't seem feasible to wire up the key container with the current TUF metadata implementation, which uses SSlibKey.

This commit re-designs the ssh key implementation to use SSlibKey as key container and a separate Verifier for verification.

See https://github.com/gittuf/gittuf/pull/429#issuecomment-2151588628 for more detailed design considerations.

Change details:

• Add an SSHKeyTypeConstant
• Rename Import to NewKeyFromFile
• Replace ssh Key with ssh Verifier, which satisfies the dsse.Verifier interface, but is otherwise a black box (no key details exposed).
• Change NewKeyFromFile to return an SSlibKey, to be included in TUF metadata.
• Add NewVerifierFromKey function to create an ssh Verifier from a corresponding SSlibKey, e.g. included in TUF metadata, to verify a signature from an ssh Signer.
• Add TUF metadata integration POC / usage example in tuf_test.go (see comment in #429).

[lukpueh]

Note that unlike #429, this PR does not add a "ssh Verifier to SSlibKey" conversion function, because with the re-design there does not seem to be a use case.

The general workflow should be:

1. Import SSlibKey from ssh key file
2. Add to TUF metadata, or cache otherwise
3. Convert Key to Verifier when verifying a related signature

[lukpueh]

Btw. creating a Signer is currently a bit cumbersome:

https://github.com/gittuf/gittuf/blob/bd5d1d715414e66910e00fa1bc0d23e642af60cb/internal/signerverifier/dsse/dsse_test.go#L65-L78

NewSignerFromFile(path string) or NewSignerFromKey(key SSLibKey, path string) functions would be helpful. I can add those in a separate PR, if there is a need.