sdk: Allow map of plugins to be passed to SDK

This adds a Plugins field to the SDK Options struct that accepts a
map[string]plugins.Factory similarly to the map created by
runtime.RegisterPlugin this map is passed to the discovery.Factories
plugin similarly, again, to how it works within the Runtime module. This
allows an OPA SDK instance to initialize plugins.

Fixes #3826

Signed-off-by: Edward Paget <edward.paget@chime.com>

Fixed changes requested by @tsandall (squash before merge)

Signed-off-by: Edward Paget <edward.paget@chime.com>