Allow filtering of headers sent in CheckRequest to gRPC ext authz

*Title*: Allow filtering of headers sent in CheckRequest to gRPC ext authz

*Description*:

The HTTP ext authz allows for the filtering of headers from the request sent to the ext authz service (`allowed_headers`: https://www.envoyproxy.io/docs/envoy/latest/api-v3/extensions/filters/http/ext_authz/v3/ext_authz.proto#extensions-filters-http-ext-authz-v3-authorizationrequest), but the gRPC version does not.

In my case a lot of non-required headers are being passed along to my ext authz, I would prefer if they were not. I could of course move to straight HTTP but I don't really want to do that either.