Remove 'external values', and replace with swapped out XContentParsers

[romseygeek]

The majority of field mappers read a single value from their positioned
XContentParser, and do not need to call nextToken. There is a general
assumption that the same holds for any multifields defined on them, and
so the XContentParser is passed down to their multifields builder as-is.
This assumption does not hold for mappers that accept json objects,
and so we have a second mechanism for passing values around called
'external values', where a mapper can set a specific value on its context
and child mappers can then check for these external values before reading
from xcontent. The disadvantage of this is that every field mapper now
needs to check its context for external values. Because the values are
defined by their java class, we can also know that in the vast majority of
cases this functionality is unused. We have only two mappers that actually
make use of this, CompletionFieldMapper and GeoPointFieldMapper.

This commit removes external values entirely, and replaces it with the ability
to pass a modified XContentParser to multifields. FieldMappers can just check
the parser attached to their context for data and don't need to worry about
multiple sources.

Plugins implementing field mappers will need to take the removal of external
values into account. Implementations that are passing structured objects
as external values should instead use ParseContext.switchParser and
wrap the objects using MapXContentParser.wrapObject().

GeoPointFieldMapper passes on a fake parser that just wraps its input data
formatted as a geohash; CompletionFieldMapper has a slightly more complicated
parser that in general wraps its metadata, but if textOrNull() is called without
the parser being advanced just returns its text input.

Relates to #56063