Bluetooth: OTS: add connect() function to ots_l2cap

Add an l2cap connect() function to OTS.  This is a function for
clients, added to prepare for the upcoming OTC client (re)using the
ots_l2cap module.

As part of this, factor out an internal function to find a free l2cap
context.

Signed-off-by: Asbjørn Sæbø <asbjorn.sabo@nordicsemi.no>