Implement API for interface credentials

Overview:
For security reasons we need to store interface credentials separately from the rest of the interface. This allows them to be managed/retrieved via separate endpoints w/ their own permissions.

New "interface_credentials" Schema:

• id (string) - UUID
• username (string) - required
• password (string) - required
• interfaceId (string) - UUID - required - FK constraint

Updated "interfaces" schema:

• remove the password and username fields

API details can be found in the API Listing Document

This API is a slight deviation from the usual endpoint pattern used in acquisitions.

• The absence of a GET by CQL endpoint is intentional. There's no need to query for credentials.
• GET/PUT/DELETE by ID are different as well. Here we're managing/viewing credentials records based on the interface's UUID, not the credential's record ID.
• Since the interfaceId is present in the POST endpoint path, and possibly also in the request body, validation is required. If these do not match, an appropriate error should be returned.

These endpoints should be added to the existing "organizations-storage.interfaces" interface.

Acceptance Criteria:

• RAML and Module Descriptors are updated
• Schemas and examples are created (new schema and updated interface schema)
• Endpoints are implemented using RMB's PgUtil class
• Unit tests are added
• API tests are added
• Investigate encryption of this table... if easy work into this story, otherwise create a follow-on story