API Endpoint to create policies

[CREATE] POST /admin/api/registry/policies
The params will be:

• 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.

Requested in David's comment in his PR and confirmed by Hery here.

A Policy is unique by its [name, version]. Once a policy is created the fields name, version and schema are frozen.