Adds fuzzer for l2cap

This adds a fuzzer for l2cap and a few related subcomponents.
Test: gd/fuzz/run bluetooth_gd_l2cap_fuzzer
Tag: #feature
Bug: 167425311
Sponsor: zachoverflow@

Signed-off-by: Dylan Katz <dylan.katz@leviathansecurity.com>
Change-Id: I052f103929749b6532c66881cf88589375cf8a5d