OpenXR loader: add API layer discovery support.

[quic-dengkail]

Loader currently did not support API layer disconvery yet. This patch supports this feature.

Loader will try to get all implicit/explicit API layers from broker supported by chosen active runtime and populate virtual API layer manifests from returned cursor. Here is authority and path used to find correct cursor:

• Loader ---> broker

  • authority:org.khronos.openxr.runtime_broker
  • path:/openxr/major_ver/abi/[abi]/api_layer/[implicit:explicit]

Corresponding patch of broker part can be found here: https://gitlab.freedesktop.org/monado/utilities/openxr-android-broker/-/merge_requests/18