sdk: expose manager options

This commit adds the possibility to configure the plugin manager with
custom options.

It will allow SDK users to override the options already provided by the
SDK and to futher customize it with configurations that were not
previously available. This is an advanced feature as it requires
some knowledge about the inner workings of OPA.

One use case for this is to provide a prometheus registerer and have the
status plugin metrics available for the client to use it in a
/metrics endpoint, for example.

resolves #6662

Signed-off-by: Francisco Rodrigues <ednofco@gmail.com>