modify proxy_get_header_map_value ABI to support returning values from multiple occurrences of the header

[mpwarres]

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.

[mpwarres]

@kyessenov FYI

[PiotrSikora]

Definitely! See: https://github.com/PiotrSikora/spec/tree/cc53262df056036427476b272fb8f2438aa7975f/abi-versions/vNEXT#proxy_get_map_values