Separate the OpenAPI spec for the Official Registry API vs. the MCP Registry API OpenAPI Schema #149

The Official Registry API should be a subset of the MCP Registry API OpenAPI Schema.

At the moment, the OpenAPI schema in the repo is trying to represent both.

For example, while the Official Registry API might specifically launch with just npm, pypi, ghcr as allowed values for registry_name, the MCP Registry API OpenAPI Schema might declare that to be an open-ended string so alternative implementations can implement whatever registries they want.