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.