Add async modifier for ProxyHttp::response_body_filter

[tqlab]

What is the problem your feature solves, or the need it fulfills?

Sometimes it is necessary to add some asynchronous operations in this method.

[pszabop]

Ditto here. I need to be able to call async logging functions, async requests out to external processes, etc.

No problem with request_body_filter, it's already asynchronous. This appears to be the lone hold out. Also the last thing I went to implement, so just noticed it after writing a pile of async code I call from request_body_filter that I can't call from response_body_filter (just for example, a logging function)

[cyberinc-ashish]

I need this too. When will this be merged to main ?