[17837] REP-2015: ROS2 DDS Security PKCS#11 Support

[Mario-DL]

This PR introduces a new REP-2015 proposal based on ros2/design/pull/332.

Note: for easily generating the html output of an .rst, the following bash macro is proposed (in the root directory of the repo):

build_rep()
{
	python3 rep2html.py $1 
	bn=${1%%.*}
	xdg-open "$bn.html" &
}

Usage example: build_rep rep-2015.rst