Callouts sample: body/header mutating sidestream

[martijneken]

A server that buffers header and body content, and sets headers based on body content.

• requires buffering the whole body
• requires FULL_DUPLEX_STREAMED mode, which requires supporting TRAILERS when upstream sends them (in this case, body chunks never have eos bit)
• server must 'chunk' the body replies to Envoy so it does not exceed the max message size of 128 KiB.

Some Envoy integration tests for full duplex:

• https://github.com/envoyproxy/envoy/blob/main/test/extensions/filters/http/ext_proc/ext_proc_integration_test.cc
• ServerWaitForBodyBeforeSendsHeaderRespDuplexStreamed
• ServerWaitForBodyAndTrailerBeforeSendsHeaderRespDuplexStreamedSmallBody