Add BLE GATT Server/emulator app

Brings up the Flipper as a connectable serial GATT peripheral via the bt service profile API and echoes what a central writes. Restores the default profile on exit.

ble_gattsrv/application.fam

@@ -0,0 +1,15 @@
+# Needs firmware from ../build.sh (full BLE stack + exported peripheral API).
+App(
+    appid="ble_gattsrv",
+    name="BLE GATT Server",
+    apptype=FlipperAppType.EXTERNAL,
+    entry_point="ble_gattsrv_app",
+    requires=["gui", "bt"],
+    stack_size=2 * 1024,
+    fap_category="Bluetooth",
+    fap_icon="icon.png",
+    fap_version="1.0",
+    fap_description="Advertise a connectable serial GATT peripheral and echo received data",
+    fap_author="andydixon",
+    fap_weburl="https://github.com/andydixon/flipper-bt-inspector",
+)