Add Map to XContentParser Wrapper

In some cases we need to parse the XContent that is already parsed into
a map. This is currently happening in handling source in SQL and ingest
node as well as parsing null_value geo values in mappings. Instead of
serialize and parse the value again or write another map-based parser
this commit adds an iterator that iterates over a map as an XContent.

Relates to elastic#43554

Author: imotov