Add a 'file.info' Conduit method

Summary:
Need this for 'arc upload' and 'arc download'.

Given a file PHID or ID, provides information about it.

Test Plan:

• Implemented 'arc upload' and 'arc download' based on 'file.info'.
• Used Conduit API console to test method.

Reviewed By: codeblock
Reviewers: codeblock, jungejason, tuomaspelkonen, aran
CC: aran, codeblock, epriestley
Differential Revision: 741