Add a helper method to parse maps of objects

[romseygeek]

We have several requests and responses that pass around maps of objects,
both multi-valued with keys acting as labels for indexes or fields (for example
in GetIndex or GetMappings), or single-valued with the label pointing to a field
(for example in queries run against a single field such as term or intervals).
All of these current have hand-rolled parsing code, because there's not an easy
way to tell ObjectParser how to handle them.

This commit adds a helper method to XContentParserUtils that, given an object
parser and a json map, will parse each value using the map key as a context and
return a map of keys to objects. You can also specify whether or not the map
should have more than one value.

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Core)

[nik9000]

There is a map method on XContentParser that takes a bunch of parameters. I don't think it is quite the same as this, but it is pretty similar. Maybe this should be on XContentParser, along side that map method?

[romseygeek]

Oops, yes, XContentParser.map() does basically everything this one does except check for singletons. I think a singleton map parser is useful though; I'll open a new PR to add a singletonMap function to XContentParser