Subsys: Bluetooth: Audio: Add presentation delay to bt_bap_unicast_group

[koffes]

Summary

When a unicast group has been created, it would be good to have an API to fetch the previously/currently used presentation delay for sink and source even if all servers in the group have disconnected.

Ref discussion with @Thalley

Describe the solution you'd like

We should add the sink PD and source PD to

zephyr/subsys/bluetooth/audio/bap_endpoint.h

Line 82 in 7390004

struct bt_bap_unicast_group {

These can then be read out by having a *_get function.

Alternatives

No response

Additional Context

No response