[sr-dev] [kamailio/kamailio] add ca_path param to tls module (#2682)

Currently tls module has `ca_list` param that

`Sets the CA list file name. This file contains a list of all the trusted CAs certificates used when connecting to other SIP implementations. If a signature in a certificate chain belongs to one of the listed CAs, the verification of that certificate will succeed.`

This issue proposes adding a new tls param `ca_path`. Its value would be a directory that contains any number of CA certificate files thus making it unnecessary to cat these files to a single `ca_list` file.

Implementation could be based on SSL_CTX_set_default_verify_dir() OpenSSL API function.