Support field_templates (macros)

[assafrabin]

Exists in original marine

[tomlegkov]

The feature:

Support aggregating multiple fields under one field. For example, when parsing a packet we don't know if it's IPv4 or IPv6 before parsing, but we know we want the source ip.

So, we'll define a new macro: macro.ip.src: [ip.src, ipv6.src]. And when Marine parses, it'll parse both ip.src and ipv6.src and choose the one that was parsed (under the macro.ip.src key). If multiple fields are parsed, choose the first one from the list.