feat: add support for custom server notifications (#580)

https://github.com/modelcontextprotocol/rust-sdk/pull/556 introduced support for
custom client notifications, so this PR makes the complementary change, adding
support for custom server notifications.

MCP clients, particularly ones that offer "experimental" capabilities,
may wish to handle custom server notifications that are not part of the
standard MCP specification. This change introduces a new
`CustomServerNotification` type that allows a client to process
such custom notifications.

- introduces `CustomServerNotification` to carry arbitrary methods/params while
  still preserving meta/extensions; wires it into the `ServerNotification` union
  and `serde` so `params` can be decoded with `params_as`
- allows client handlers to receive custom notifications via a new
  `on_custom_notification` hook
- adds integration coverage that sends a custom server notification end-to-end
  and asserts the client sees the method and payload

Test:

```shell
cargo test -p rmcp --features client test_custom_server_notification_reaches_client
```

crates/rmcp/src/model/meta.rs

@@ -4,8 +4,8 @@ use serde::{Deserialize, Serialize};
 use serde_json::Value;
 
 use super::{
-    ClientNotification, ClientRequest, CustomClientNotification, Extensions, JsonObject,
-    JsonRpcMessage, NumberOrString, ProgressToken, ServerNotification, ServerRequest,
+    ClientNotification, ClientRequest, CustomNotification, Extensions, JsonObject, JsonRpcMessage,
+    NumberOrString, ProgressToken, ServerNotification, ServerRequest,
 };
 
 pub trait GetMeta {
@@ -18,7 +18,7 @@ pub trait GetExtensions {
     fn extensions_mut(&mut self) -> &mut Extensions;
 }
 
-impl GetExtensions for CustomClientNotification {
+impl GetExtensions for CustomNotification {
     fn extensions(&self) -> &Extensions {
         &self.extensions
     }
@@ -27,7 +27,7 @@ impl GetExtensions for CustomClientNotification {
     }
 }
 
-impl GetMeta for CustomClientNotification {
+impl GetMeta for CustomNotification {
     fn get_meta_mut(&mut self) -> &mut Meta {
         self.extensions_mut().get_or_insert_default()
     }
@@ -104,7 +104,7 @@ variant_extension! {
         ProgressNotification
         InitializedNotification
         RootsListChangedNotification
-        CustomClientNotification
+        CustomNotification
     }
 }
 
@@ -117,6 +117,7 @@ variant_extension! {
         ResourceListChangedNotification
         ToolListChangedNotification
         PromptListChangedNotification
+        CustomNotification
     }
 }
 #[derive(Debug, Serialize, Deserialize, Clone, Default, PartialEq)]

crates/rmcp/src/model/serde_impl.rs

@@ -3,7 +3,7 @@ use std::borrow::Cow;
 use serde::{Deserialize, Serialize};
 
 use super::{
-    CustomClientNotification, Extensions, Meta, Notification, NotificationNoParam, Request,
+    CustomNotification, Extensions, Meta, Notification, NotificationNoParam, Request,
     RequestNoParam, RequestOptionalParam,
 };
 #[derive(Serialize, Deserialize)]
@@ -249,7 +249,7 @@ where
     }
 }
 
-impl Serialize for CustomClientNotification {
+impl Serialize for CustomNotification {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where
         S: serde::Serializer,
@@ -277,7 +277,7 @@ impl Serialize for CustomClientNotification {
     }
 }
 
-impl<'de> Deserialize<'de> for CustomClientNotification {
+impl<'de> Deserialize<'de> for CustomNotification {
     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where
         D: serde::Deserializer<'de>,
@@ -294,7 +294,7 @@ impl<'de> Deserialize<'de> for CustomClientNotification {
         if let Some(meta) = _meta {
             extensions.insert(meta);
         }
-        Ok(CustomClientNotification {
+        Ok(CustomNotification {
             extensions,
             method: body.method,
             params,