Make RustMatrixClient.close asynchronous (#4513)

* Make `RustMatrixClient.close` asynchronous

This is a safer way to destroy the Rust instances associated to it. Since `MatrixClient` doesn't implement `Closeable` anymore, the method has been renamed to `destroy` to follow the existing naming in the project.

libraries/matrix/test/src/main/kotlin/io/element/android/libraries/matrix/test/FakeMatrixClient.kt

@@ -181,8 +181,6 @@ class FakeMatrixClient(
         deactivateAccountResult(password, eraseData)
     }
 
-    override fun close() = Unit
-
     override suspend fun getUserProfile(): Result<MatrixUser> = simulateLongTask {
         val result = getProfileResults[sessionId]?.getOrNull() ?: MatrixUser(sessionId, userDisplayName, userAvatarUrl)
         _userProfile.tryEmit(result)