Fix dynamic values and object handling in subobjects: auto

[kkrik-es]

Objects and fields with flattened names may not be detected during document parsing, depending on the use of curly brackets for intermediate objects. In this case, dynamic mappers get created for leaf values that may not match the ones in the mapping, e.g. long vs integer. This gets fixed by checking for existing mappers for a given leaf path on value retrieval.

More so, dynamic (empty) objects may be created to match intermediate objects that would otherwise be flattened. For instance, parsing "foo": { "bar": { "baz": 10 } } may create a dynamic object foo before realizing that there's an existing object foo.bar containing baz. In these cases, the intermediate object should be dropped.

Also, split 20_synthetic_source.yml by moving the copy_to tests in a separate file.

Related to ##108826

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[kkrik-es]

The bwc failure is due to the added test, fails on 8.16 until this get backported..