Create admin tool to deal with MoodleNet communication (tool_moodlenet all versions)

This issue is a foundation for several others as it creates the admin tool plugin (tool_moodlenet) and the basic communication flow between MoodleNet and Moodle. More specifically, it provides the endpoint which MoodleNet can POST information about the file (as JSON) to.

This issue will help us to complete the following user story (it won't fully complete it because it only deals with the endpoint, not the processing + decision code):

Acceptance criteria:

• There is a new admin tool 'tool_moodlenet' created in core.
• The plugin provides an endpoint for MoodleNet to POST to
• MoodleNet should be able to POST to this endpoint (we need to simulate this in in testing in the first instance as it isn't built yet), regardless of whether a user is logged in or not and the POST data should be retained after login for use by the tool.
• The referring MoodleNet page should be captured and present as a param in the wantsurl.

We require a plugin (admin tool) to handle receiving requests to include MoodleNet content into a course.

It has been suggested that the workflow could be similar to how plugins are installed from the plugin database (moodle.org).
Plugin workflow - The button to install a plugin is a base64 encoded string which contains information (name, component, version) related to the plugin to make a request back to fetch it later. - End plugin workflow.

• Checks need to be done as to whether accepting files from MoodleNet is allowed by the system.
• The user needs to be logged in with an authenticated account.
• After getting the file it then has to be decided what to do with it
  • Backup files to ultimately be restored.
  • Other file types to be handled with a call back to activity plugins.
  • An option to just put the file into the private files area of the user.