modify proxy_get_header_map_value ABI to support returning values from multiple occurrences of the header #53
Description
The proxy_get_header_map_value hostcall currently only returns a single value, which doesn't accomodate headers that appear multiple times, such as Set-Cookie.
The next Proxy-Wasm ABI minor version should update proxy_get_header_map_value or add a variant that supports returning multiple values.