MCP servers, particularly ones that offer "experimental" capabilities,
may wish to handle custom client notifications that are not part of the
standard MCP specification. This change introduces a new
`CustomClientNotification` type that allows a server to process
such custom notifications.
- introduces `CustomClientNotification` to carry arbitrary methods/params while
still preserving meta/extensions; wires it into the `ClientNotification` union
and `serde` so `params` can be decoded with `params_as`
- allows server handlers to receive custom notifications via a new
`on_custom_notification` hook
- adds integration coverage that sends a custom client notification end-to-end
and asserts the server sees the method and payload
Test:
```shell
cargo test -p rmcp --features client test_custom_client_notification_reaches_server
```
* feat: provide more context information
1. provide context for notification
2. allow extract more info from tool call
3. inject http request part for streamable http server
* docs: add document for getting peer from context
* feat: better http server support
1. allow user get extensions in tool call.
2. allow user serve sse service without initialization.
* fix: fix document test
