Authority-mode keys and certificates should be owned by dirauth module

Right now these functions are defined in the relay module, when only dirauths need them:

• get_my_v3_authority_cert
• get_my_v3_authority_signing_key
• get_my_v3_legacy_cert
• get_my_v3_legacy_signing_key

We should move them along with related fields and support functions, to feature/dirauth.