Forward SspRequest pairing event to callback

This forwards SspRequest to callbacks and implements the client to
handle displaying of passcode.

Bug: 188718349
Tag: #floss
Test: manual - Build floss on Linux, run btclient and create_bond to an
LE keyboard

Change-Id: I6b0b3ac1ba1e4c1e03405deb360bb8f2b0ead2c1