Add Map to XContentParser Wrapper (#44036)

In some cases we need to parse some XContent that is already parsed into
a map. This is currently happening in handling source in SQL and ingest
processors as well as parsing null_value values in geo mappings. To avoid
re-serializing and parsing the value again or writing another map-based
parser this commit adds an iterator that iterates over a map as if it was
XContent. This makes reusing existing XContent parser on maps possible.

Relates to #43554

Author: imotov