add http server test

[helight]

1. Add an http server for testing http filter.
2. Add config.yaml for testing http filter.
3. Add readme for testing http filter instruction.

[helight]

@lizan please help to review this pr

[lizan]

Can we have some sort of test for this config? Like verify examples in envoy repo.

[lizan]

nit: keep this lowercase

[helight]

This is just for testing this function
http_filter.cc

const LowerCaseString HttpSampleDecoderFilter::headerKey() const {
  return LowerCaseString(config_->key());
}

[helight]

Can we have some sort of test for this config? Like verify examples in envoy repo.

Ok

[helight]

Can we have some sort of test for this config? Like verify examples in envoy repo.

@lizan I add a startup.sh and verify.sh, please help to check