ROS2 DDS Security PKCS#11 URI support

[IkerLuengo]

The DDS-Security specification defines the use of Hardware Security Modules (HSM) and PKCS#11 URIs as an alternative to private keys and certificates stored in the file system. Current implementation only supports these tokens to be directly stored in the file system as .pem files. This is a design proposal to support PKCS#11 URIs.

The changes affect to the RMW implementations, as these are filling the DDS security attributes for the participant. However, it also affects the contents of the enclave directories in the keystore. Although the proposed changes are totally backwards compatible (meaning that current RMW implementations will continue working if no PKCS#11 URIS are used), description of the new enclave contents and the expected RMW behavior seems appropriate.

[clalancette]

@ros2/security_working_group Thoughts about this proposal?

[ros-discourse]

This pull request has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/ros-2-tsc-meeting-minutes-2021-08-19/22008/1

[IkerLuengo]

To give some context, this proposal was discussed an approved on the @ros2/security_working_group on June (minutes here, although with Sid gone, they are still unmerged).

[MiguelCompany]

@quinz @fujitatomoya @asorbini I've open #332 since I don't have write access on the repo of the original contributor.

This PR can be closed.

[clalancette]

Closing based on the last comment.