Support file:// schema when adding servers

[cmrigney]

What I did

Adds support for adding a server from a yaml/json file like so --server file://my-server.yaml. The format of this file can either be a full catalog yaml/json like the one output when you run task catalog -- my-server in mcp-registry, or the file can just be a single server entry directly.

Example

Create duckduckgo.yaml

name: duckduckgo
title: DuckDuckGo
type: server
image: mcp/duckduckgo@sha256:68eb20db6109f5c312a695fc5ec3386ad15d93ffb765a0b4eb1baf4328dec14f
description: A Model Context Protocol (MCP) server that provides web search capabilities through DuckDuckGo, with additional features for content fetching and parsing.
allowHosts:
  - html.duckduckgo.com:443

Then create the catalog:

docker mcp catalog-next create duckduckgo-catalog --title "DuckDuckGo Catalog" --server file:///duckduckgo.yaml

(not mandatory) A picture of a cute animal, if possible in relation to what you did

[cmrigney]

Just an incidental, but unrelated fix!

[slimslenderslacks]

this works for adding servers to existing catalogs too, right?

[cmrigney]

@slimslenderslacks yep it should! You just need to compose the right command I believe.

[cmrigney]

So to "add" one to an existing catalog, you'd basically select from the existing catalog on create and add more --servers to it:

docker mcp catalog-next create duckduckgo-catalog --title "DuckDuckGo Catalog" --server catalog://duckduckgo-catalog/duckduckgo --server file:///another-server.yaml

Which makes me think we need to be able to select all from a catalog, e.g. --server catalog://duckduckgo-catalog/*. But that's unrelated to this PR.