Serve ccache metrics and other utilities through api-exec

Summary:
D3492 has introduced metrics for ccache as a command line utility.

That would have been enough in the 90s where cgi-bin was a reality,
but nowadays we don't even deploy Apache anymore.

This change solves this issue through api-exec, a tiny Flask
application to provide a request <> command gateway.

To reduce security issues, the gateway DOES NOT try to inject
URL parameters as shell arguments. It runs known commands and
arguments without any customization in the URL.

Ref T2039

Test Plan:
Deployed on WindRiver.

Metrics for ccache available at http://172.27.27.35:2337/metrics/ccache

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T2039

Differential Revision: https://devcentral.nasqueron.org/D3494