hcm: Allow filter to control on emitting trailers (e.g. to skip encoding trailers when empty)

Context: Some of the filters, e.g. gRPC Web requires to skip encoding trailers due to https://github.com/envoyproxy/envoy/issues/10514.

i.e. currently, we have the following "problematic" response frames.

```
[ 5.036] recv DATA frame
[ 5.040] recv DATA frame
[ 5.053] recv HEADERS frame
; END_STREAM | END_HEADERS
```

What we want is something like:

```
[ 5.036] recv DATA frame
[ 5.040] recv DATA frame
; END_STREAM
```

We have the current data frames since we always encode trailers. The proposed solution will be to have control from the filter side.

A couple of options:
1. introduce a new return enum to `FilterTrailersStatus`, to indicate that trailers should not be emitted.
1. add `end_stream` flags to `addEncodedData`, end stream there and `encodeTrailers` return `StopIteration`

From the discussion [here](https://github.com/envoyproxy/envoy/pull/12178#discussion_r459932842), the second of the options above is preferable and when doing that we should do a parallel fix in `addDecodedData` too.