Add anonymous version reporting feature to OPA

This commit updates the OPA `run` and `version` commands to report the
version of the running OPA instance to an external service.

In case of the `opa run` command, this feature is ON by-default and
can be disabled using the --skip-version-check flag. In the server mode,
reports are sent periodically while in repl mode only once at start-up.

In case of the opa version command, this feature can be enabled by
specifying the --check or -c flag.

Reports are sent to the configurable external service
on a best-effort basis.

Fixes #1253

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>