ESQL: more test coverage for union types

Not being able to use small numeric types like short and byte as union types indicates that we need to expand test coverage for union types.

Let's add:

[ ] integration tests where we perform from index* | eval x = to_integer(field) (resp. to_string, to_ip etc.) for 2 - 3 indices with field mapped to different types (all types that work for a given conversion function like to_integer); also tests with subfields, i.e. when field is replaced by parent.subfield.subsubfield; there's already known problems with gauge fields https://github.com/elastic/elasticsearch/issues/111687.
[ ] fixes for whatever we find out doesn't work in the added tests, for instance https://github.com/elastic/elasticsearch/issues/111277
[ ] a couple of analyzer tests that demonstrate a) the field attributes in the EsRelation after analyzing a plan with different union types and b) the projection we add at the top of the plan to project out the added synthetic attributes
[ ] potentially a couple more csv tests that look for edge cases with keep/drop, and that use union type conversions like client_ip::ip in different, subsequent stats/eval commands

elastic / elasticsearch