plugins/bundle: Support for saving and reading bundles from disk

This commit adds support to persist and load bundles from disk.
A new field is introduced in OPA's bundle configuration that can
be optionally set to enable OPA to write and read bundles from disk.
This feature will allow OPA to serve policy decisions in scenarios
such as OPA being unable to communicate with the bundle server.

Fixes #2097

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