GRPCRouteFilter Request HTTPHeaderFilter for GRPCRouteRule

[mpstefan]

As a user of NGF with applications that require GRPC traffic
I want NGF to support a GRPCRouteRule that can handle a request HTTPHeaderFilter
So that I can append specific headers required by my applications.

Acceptance

• A user can specify a GRPCRouteFilter in their GRPC routes.
• The GRPCRouteFilter can be of type "RequestHeaderModifier" as defined here.
• A user can specify a request HTTPHeaderFilter in their GRPCRouteFilter which works as specified here.

[mpstefan]

Can we leverage the existing HTTPHeaderFilter for HTTPRouteFilters here?

responseHeaderModifier is marked as Extended in the GRPCRouteFilter, but Core in the GRPCRouteFilter types. Do we implement?

[ciarams87]

@mpstefan I'm going to address requestHeaderModifier only as part of this ticket, as support for responseHeaderModifier for HTTPRoute (which 95% covers the support for GRPCRoute) is already covered in #1397. I think that ticket can probably be used to track responseHeaderModifier filter support for both Route types (or maybe another ticket would be better, but if so, #1397 should still go first).