Camera: Allow synthetic extension metadata

Synthetic metadata such as 'STATISTICS_FACES'
could be of value to extension clients.
Allow extension implementations to advertise support
for synthetic keys.

Bug: 235330843
Test:
atest -c -d
cts/tests/camera/src/android/hardware/camera2/cts/CameraExtensionCharacteristicsTest.java
atest -c -d
cts/tests/camera/src/android/hardware/camera2/cts/CameraExtensionSessionTest.java

Change-Id: I80133e4559610ecad714afa93400c00b2b6685fa