[Task] Add server function to decrypt payload

[kmcginnes]

Add a new endpoint to the proxy server to decrypt the given value using the configured private key.

Risk

When this is called by the user loading a shared URL, we will need to use the proxy server definition of the active connection. It is possible that active connection does not use the proxy server, or uses a different proxy server (unlikely, but possible). This will cause the loading of the URL to fail.