Add read,write callbacks to characteristics

During Fast pair handshake, the seeker writes to GATT characteristics, the
provider reads the value and sends the response.
We need callbacks for reading and writing GATT characteristics to test the
handshake. Static GATT DB entries are not sufficient.

PiperOrigin-RevId: 527958868

internal/platform/implementation/ble_v2.h

@@ -27,6 +27,7 @@
 #include "absl/container/flat_hash_map.h"
 #include "absl/functional/any_invocable.h"
 #include "absl/status/status.h"
+#include "absl/status/statusor.h"
 #include "absl/strings/string_view.h"
 #include "internal/platform/byte_array.h"
 #include "internal/platform/cancellation_flag.h"
@@ -284,6 +285,9 @@ struct ClientGattConnectionCallback {
 
 // Callback for asynchronous events on the server side of a GATT connection.
 struct ServerGattConnectionCallback {
+  using ReadValueCallback =
+      absl::AnyInvocable<void(absl::StatusOr<absl::string_view> data)>;
+  using WriteValueCallback = absl::AnyInvocable<void(absl::Status result)>;
   // Called when a remote peripheral connected to us and subscribed to one of
   // our characteristics.
   absl::AnyInvocable<void(const GattCharacteristic& characteristic)>
@@ -293,6 +297,24 @@ struct ServerGattConnectionCallback {
   // characteristics.
   absl::AnyInvocable<void(const GattCharacteristic& characteristic)>
       characteristic_unsubscription_cb;
+
+  // Called when a gatt client is reading from the characteristic.
+  // Must call `callback` with the read result.
+  // When a characteristic has a static value set with
+  // `GattServer::UpdateCharacteristic()`, then reading from the characteristic
+  // yields that static value. The read callback is not called.
+  // Otherwise, the gatt server calls the read callback to get the value.
+  absl::AnyInvocable<void(const BlePeripheral& remote_device,
+                          const GattCharacteristic& characteristic, int offset,
+                          ReadValueCallback callback)>
+      on_characteristic_read_cb;
+
+  // Called when a gatt client is writing to the characteristic.
+  // Must call `callback` with the write result.
+  absl::AnyInvocable<void(const BlePeripheral& remote_device,
+                          const GattCharacteristic& characteristic, int offset,
+                          absl::string_view data, WriteValueCallback callback)>
+      on_characteristic_write_cb;
 };
 
 // A BLE GATT client socket for requesting GATT socket.