Add proof to schema for credentials intended for issuance

[aljones15]

• [x] Check to ensure minor changes to res-oas.js did not cause any regressions

Features:

• Adds support for type: array to oneOf schemas
• Adds proof to issue credential API endpoint schema
• Redefines UnsecuredCredential in components to allow it to optionally have a proof

Addresses some, but not all of this issue: https://github.com/w3c-ccg/vc-api/issues/422