Expose global account data get and set on MatrixClient

MatrixClient had no way to read or write global account data although the
underlying client supports it.

Add getAccountData and setAccountData to the interface and implement them in
RustMatrixClient using the same withContext/runCatchingExceptions idiom as the
neighbouring methods. The content is the raw JSON string the SDK returns and
accepts: event types are open ended, so a typed wrapper belongs to whichever
caller needs one. Scope is limited to global account data; the room level
variant and the change notification APIs can follow when a feature needs them.

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/MatrixClient.kt

@@ -210,6 +210,16 @@ interface MatrixClient {
      */
     suspend fun addRecentEmoji(emoji: String): Result<Unit>
 
+    /**
+     * Returns the raw JSON content of the global account data event of type [eventType], or null if it is not set.
+     */
+    suspend fun getAccountData(eventType: String): Result<String?>
+
+    /**
+     * Sets the global account data event of type [eventType] to the raw JSON [content].
+     */
+    suspend fun setAccountData(eventType: String, content: String): Result<Unit>
+
     /**
      * Marks the room with the provided [roomId] as read, sending a fully read receipt for [eventId].
      *