support custom PLC endpoints #23

Currently, the base of PlcSource::PlcDirectory is private, so we can't use custom PLC endpoints like plc directory mirror or custom plc directory for sandboxed environment.

The base should be exposed.

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum PlcSource {
    /// Use the public PLC directory
    PlcDirectory {
        /// Base URL for the PLC directory
        base: Url,
    },
    /// Use the slingshot mini-docs service
    Slingshot {
        /// Base URL for the Slingshot service
        base: Url,
    },
}