Consistent style in the ABI (expanded vs minimal)

[PiotrSikora]

During the initial development, we went through a lot of back-and-forth both expanding and minimizing the ABI surface (e.g. get_buffer_bytes(buffer_type) vs get_http_request_body_chunk() and similar, set_shared_data(NULL) vs remove_shared_data()), both internally and during Envoy upstreaming process.

The version that was initially merged into Envoy, and which the ecosystem ossified around, ended up with some unfortunate inconsistencies (using both: expanded proxy_{get,add,replace,remove}_header_map_value, and minimal proxy_set_buffer_bytes(start, offset) or proxy_set_shared_data(NULL)).

We should clean this up and stick to one style.

While I don't have strong preference one way or the other, note that splitting features into groups (#57) and having specialized context types with their own hostcalls (#9) probably means that we should be expanding the ABI surface instead of minimizing it.

@mpwarres @martijneken @leonm1 @wbpcode @thibaultcha any thoughts?