Lazy evaluation of compression decision in compressor_filter for http response w/o content-length

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*Title*: *One line description*
Buffer response header in **compressor_filter** and perform compression decision when receiving first data frame (encode_data)

*Description*:
>Describe the issue.

Snap leverages on **compressor_filter** to perform response compression for gRPC responses. gRPC server doesnt add **content-length** to response header that the compressor filter will compress small response payload as well , which causes some overhead in egress bandwidth.

What Snap does(already running in prod env) is to perform a lazy evaluation when receiving first data frame instead of buffering all data frames.

Snap want to seek some feedback from envoy community that
1. this operation is feasible (Snap can create a draft PR to review later)
2. upstream the change to envoy

[optional *Relevant Links*:]
>Any extra documentation required to understand the issue.