Relax concat args to allow any ExpressionArg (#48506)

* Relax the arg types to ExpressionArg for concat expressions in the legacy schema

Relax the arg types to ExpressionArg for concat since many DBs allow to concatenate non-string types. This also aligns
with the corresponding MLv2 schema and with the reference docs we publish.

Fixes #39439

* Add nested concat schema tests

* Add nested-concat query-processor tests