Add support for on_{request,response}_meta_data.

[mathetake]

just like c++ SDK https://github.com/proxy-wasm/proxy-wasm-cpp-sdk/blob/7afb39d868a973caa6216a535c24e37fb666b6f3/proxy_wasm_api.h#L460-L462

[Echo-Van]

Is there any latest progress in this work?

[sanfern]

Is there any latest progress in this?

[mathetake]

@sanfern this is fairly simple to implement so I intentionally deprioritize this in my list so that others can contribute. We welcome your contributions.

[rtttech]

@sanfern this is fairly simple to implement so I intentionally deprioritize this in my list so that others can contribute. We welcome your contributions.

Need this feature too.

[mathetake]

#451