Dynamic modules are unable to add response trailers if the incoming response has no trailers

*Title*: *Dynamic modules are unable to add response trailers if the incoming response has no trailers*

*Description*:
> In a dynamic module, calls to `envoy_filter.set_response_trailer(...)` fail (no trailer set, function returns `false`) if either the incoming response has no trailers to begin with (e.g. an http/1.1 response), or the call is made before trailer parsing has occurred (e.g. in `on_response_body` hook).

*Repro steps*:
1. Set up a dynamic module filtering a request that is made with http/2 (e.g. grpc) but the upstream cluster responds with http/1.1
2. in `on_response_body` attempt to set a trailer
3. see that the call returns `false` and no trailers are set.

I'll set up a minimal repro when I get a chance, just noting the current state so I can link my PR to an issue