Remove support for 'external values' in document parsing?

For fields whose value can be a JSON array or object like geopoints, the standard way of supporting multi-fields doesn’t work. When parsing a document, the parent mapper consumes the whole complex value, so when the parser is passed on to its subfields, they aren't able to parse the source value. To help support this case, parent fields can pass a parsed object through `ParseContext#externalValue`. Subfields can then use this parsed object instead of parsing the source value.

This support for 'external values' has a few downsides:
* It makes it harder to understand what data ended up getting indexed for a field, since a parent field is allowed to supply literally anything as an 'external value'. This in turn makes it hard to consult the _source to find field values (as we hope to do in #55363).
* In `FieldMapper#parseCreateField`, each field mapper must check whether an external value is set, regardless of whether it makes sense to pass custom values to the mapper. Making sure to check for an external value is a silent contract and is easy to forget.

I wanted to raise the idea of removing support for external values. Within our own code, here are the mappers that would be affected:
* `ParentJoinFieldMapper` passes the IDs to its internal `ParentIdFieldMapper` objects. This could be replaced by a custom method on `ParentIdFieldMapper`.
* `PercolatorFieldMapper` passes the query builder to an internal `BinaryFieldMapper`. This could also be replaced by a custom method.
* `CompletionFieldMapper` passes a parsed completion object to its sub-fields (#34081). I wonder how important it is to support multi-fields for completion mappings, or if we could just drop support.
* `GeoPointFieldMapper` passes a geohash to its sub-fields. This allows it to have multi-fields that are geopoints or even keywords. I also wonder about the use cases + importance of this functionality.

It looks like external values were also used in the attachment mapper, but this mapper was removed and migrated to an ingest processor.

Here is a proposed process for removing this support
- [x] remove the use of external value passing in ParentJoin and Percolator mappers (#71834)
- [x] remove external values on ParserContext entirely, and replace on completion and geo_point with a faked XContentParser (#72203)
- [ ] deprecate and remove multifields on geo_point
- [ ] deprecate and remove nested completion mappers
- [ ] remove `ParserContext.switchParser()`