Add external policy mode

[jonnydgreen]

Closes: #18

[jonnydgreen]

Before I start the documentation and types etc I thought it would be good to get consensus on what I've done so far :) This PR implements as external policy mode as follows:

• Define two new modes of operation:
  • directive (default) - unchanged
  • external-policy (default)
    • Define an auth policy:
      • On fields and types
      • Defined field policy is available as the first parameter of applyPolicy function
    • Works in both gateway and server mode
• Existing functionality/options unaffected
• Refactored the code so that the directive mode builds an external policy which is then registered as normal. As a result, these all share the same logic.

Are additional features/changes you'd like to look into for this? Let me know what you think :)

[mcollina]

looks good!!

[jonnydgreen]

Awesome! I'll sort out the final bits :)

[mcollina]

lgtm