Support Opus as A2DP vendor codec.
The specification for vendor A2DP codec is our Pipewire-specific one, so
it is compatible only with devices running Pipewire.
Change codec factory names to api.codec.bluez5.*, so that they won't
conflict with old config file lib name rules for api.bluez5.*
Specify the fallback library name when loading the codecs, so that it
works without the rules in config files.
Make easier to package A2DP codecs separately, by splitting each to a
separate SPA plugin. Adjust the code to not use a global variable for
the codec list.
The A2DP SPA interface API is in the bluez5 private headers, and not
exposed in installed SPA headers, as it's too close to the
implementation.
