Support MCP's server.json and mcp.json import

goose has its own repository for MCP servers but given that there is an MCP server for everything now, this concept is outdated. goose has deeplinks to install remote MCP servers but uses its own format for that. Meanwhile the world has standardized around mcp.json as an unofficial standard, with [server.json](https://github.com/modelcontextprotocol/registry/blob/main/docs/reference/server-json/generic-server-json.md) as an official standard. We should [retire our registry](https://github.com/aaif-goose/goose/discussions/10830) and adopt that standard, support it through deeplinks, upload & paste. Integration with registries like Pulse should then be relatively easy and can be done as part of this or in a follow up.