Incorporate MCP clients into the official MCP registry

[tadasant]

While the initial scope of the official registry is of course MCP server-oriented, there is a (less pressing) need for similar functionality for MCP clients.

Accomplishing this would allow us to deprecate the manually managed list here.

Things to think through:

• Does adding clients create issues for the core registry use case of being a server registry?
• "client" vs. "host"
• How should we model it in the OpenAPI spec?
• Do we need some sort of client.json concept?
• What implementation work needs to be done?