Implement JSON $schema property in Agent

[wsobel]

Add support in Agent for $schema reference in all the JSON documents.

• Reference the scemas.mtconnect.org site by default
• Allow for detection of files if a directory is given in the Files section.
• Allow for override with a URI mapped to a file or location.

[wsobel]

The agent changes are complete. User validation is required. This will go into release 2.7.0.11