API Endpoint to show custom policies

[SHOW] GET /admin/api/registry/policies/:id

Returned values should include:

• id Integer identifier of the policy record in System
• name String which corresponds to the directory name of the policy.
• version String corresponding to the version in the schema.
• schema the String representation of the JSON schema. We will validate it upon creation.

id can be either an actual system id or a string in the format name-version.