Bluetooth: controller: Introduce preferred central conn spacing config

Adds configuration BT_CTLR_SCHED_ADVANCED_CENTRAL_CONN_SPACING to enable
per-application preferred central connection spacing. This is an
extension of feature BT_CTLR_SCHED_ADVANCED, which when enabled, adds
user defined spacing between existing, and new central initiated
connections.

This is crucial for ensuring that centrals acting as ACL connection
based multi-channel streaming sources do not get overlapping
connections, and that spacing allows full frame transfer without
scheduling collision.

Due to the simple, compile-time configuration, use is limited to
specific applications in which the specified spacing is required.

Signed-off-by: Morten Priess <mtpr@oticon.com>