Add injectDataToFilterChain(data, end_stream) methods to NetworkFilter callbacks

[yskopets]

Title: Similarly to HTTP use case, add injectDataToFilterChain(data, end_stream) methods to NetworkFilter callbacks

Description:

Recently, new methods have been added to StreamFilterCallbacks in order to support low bandwidth simulation (see #6242):

• StreamDecoderFilterCallbacks::injectDecodedDataToFilterChain(data, end_stream)
• StreamEncoderFilterCallbacks::injectEncodedDataToFilterChain(data, end_stream)

It would be useful to have methods with the same semantics in the context of NetworkFilters as well.

Example use cases include:

• low bandwidth simulation at L4 (see Slow network / local rate limit HTTP filter #5942)
• piping request/response data through an out-of-process filter (see Add ability to execute Filters over gRPC to mutate payload. #5209)

[mattklein123]

@yskopets thanks for opening, I just checked and yes, you are correct, we will need these new APIs to do similar things as what we are doing for HTTP. Conveniently, the implementation should be much easier. Will mark help wanted.

[yskopets]

@mattklein123 Great! I will try to implement it