Expose 'append' parameter in Pusher::set() for multi-profile push notification support

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## Summary

The `append` parameter for the `/_matrix/client/v3/pushers/set` endpoint is not exposed through the SDK's `Pusher::set()` method or the FFI bindings. This parameter is necessary for multi-profile/multi-account support where multiple Matrix accounts on the same device need to share a single APNS/FCM push token.

## Use Case

In our iOS app, we support multiple Matrix accounts logged in simultaneously. Each account needs to register for push notifications using the same device push token. Without the `append: true` flag, registering a pusher for one account removes the pusher for other accounts that share the same `pushkey` and `app_id`.

## Current Workaround

We currently bypass the SDK and make direct HTTP calls to the Matrix API:

```swift
// We have to make direct HTTP calls because the SDK doesn't expose the append flag
let pushersSetURL = homeserverURL.appendingPathComponent("_matrix/client/v3/pushers/set")
var request = URLRequest(url: pushersSetURL)
request.httpMethod = "POST"
// ... manually construct JSON body with "append": true
```

## Technical Details

The `append` flag already exists in Ruma's `PusherPostData`:

```rust
pub struct PusherPostData {
pub pusher: Pusher,
pub append: bool, // Already available!
}
```

However, the SDK's convenience method `Request::post(pusher)` hardcodes `append: false`, and neither the core SDK nor the FFI bindings expose this parameter.

## Proposed Solution

Add the `append` parameter with a default value of `false` to preserve backward compatibility:

1. **crates/matrix-sdk/src/pusher.rs** - Add `append` parameter with default:
```rust
pub async fn set(&self, pusher: Pusher, append: bool) -> Result<()> {
let action = PusherAction::Post(PusherPostData { pusher, append });
self.client.send(set_pusher::v3::Request::new(action)).await?;
Ok(())
}
```

2. **bindings/matrix-sdk-ffi/src/client.rs** - Add `append` to FFI with default `false`:
```rust
pub async fn set_pusher(
&self,
identifiers: PusherIdentifiers,
kind: PusherKind,
app_display_name: String,
device_display_name: String,
profile_tag: Option,
lang: String,
append: bool, // Default to false for backward compatibility
) -> Result<(), ClientError>
```

By defaulting `append` to `false`, this change preserves the existing behavior for all current consumers while enabling multi-account support for those who need it.

## References

- Matrix spec for pushers: https://spec.matrix.org/v1.6/client-server-api/#post_matrixclientv3pushersset
- The `append` field: "If true, the homeserver should add another pusher with the given pushkey and app id in addition to any others with different user ids."

Start in crates/matrix-sdk/src/pusher.rs and inspect how Pusher::set builds PusherPostData and calls the pusher endpoint. Then review bindings/matrix-sdk-ffi/src/client.rs and the existing FFI set_pusher entry point. Done means append can be supplied through both APIs while retaining false behavior for existing callers.

Written by the indexing model from the issue text.