Move key filtering logic into key_manager.rs

[thobson88]

Some key management logic is currently in the wrong place, in particular:

• in the trustchain-http crate, the issue_credential() method inside issuer.rs
• in the trustchain-ion crate, the signing_key() method inside attestor.rs.

This logic should be moved into the KeyManager or the AttestorKeyManager (in trustchain-core).