Add support for CA certificate file in addition to a CA directory

[amadio]

Davix supports setting a custom CA directory via addCertificateAuthorityPath(). However, it seems that overriding the CA with a file instead of a directory is not supported. In XRootD, the variable X509_CERT_FILE is used to set this. It would be great if Davix implemented the same functionality so that xrdcl-http would automatically work when that variable is set. See also xrootd/xrootd#2136. Support for the CA path is done here in XRootD. If an addCertificateAuthorityFile() is provided by Davix, we could use it in the same place to set the file path.