Add support for variables in `remotes`

[thomas-sickert]

Closed by #570

Is your feature request related to a problem? Please describe.
I'm working on the GitHub MCP registry and we have two partners that have remotes, but use a generated URL for the remote. When the user installs the server via our install links in VS Code, they would need to enter this generated URL for the remote. However, the remote in the spec does not support variables, so we can't support this use case. We also have cases where a desktop app is running locally, but it's treated as a remote and the local port needs to be set as a variable.

Describe the solution you'd like
Like packages via InputWithVariables, I'd love to see remotes support variables.

Describe alternatives you've considered
Can't think of any alternatives outside of users manually changing the server configuration after install, but we'd like to have first class support for this use case in clients.

Additional context

For example, the sse transport in the schema:

{
  "SseTransport": {
    "type": "object",
    "required": [
      "type",
      "url"
    ],
    "properties": {
      "type": {
        "type": "string",
        "enum": [
          "sse"
        ],
        "description": "Transport type",
        "example": "sse"
      },
      "url": {
        "type": "string",
        "format": "uri",
        "description": "Server-Sent Events endpoint URL",
        "example": "https://mcp-fs.example.com/sse"
      },
      "headers": {
        "type": "array",
        "description": "HTTP headers to include",
        "items": {
          "$ref": "#/definitions/KeyValueInput"
        }
      }
    }
  }
}