Add Custom GUC for PXF_BASE to C extension

[axue-broadcom]

This PR adds a custom PXF guc called pxf.pxf_base. The default value for this GUC is set to empty string and can only be set via gpconfig --change pxf.pxf_base --value <PXF_BASE_DIR> --skipvalidation. Setting this guc requires a configuration reload gpstop --u.

Upon first use of the extension in a new session, the GUC will be loaded and a directory validation check will occur. A warning will be printed if the directory does not exist.