Improvements to `fetch.unionWith` API

[homedirectory]

Description

Method fetch.unionWith dates all the way back to 2012 and it is time to improve it.
Although this method is rarely used in end applications, when it is used, it often requires explicit type casts.

• 1. Change the type signature of unionWith so that the union of two fetch<T> results in the same type fetch<T>.

  This is a breaking change, as it may cause compilation errors.

• 2. Provide an alternative method for dynamic use cases where one or both fetch models are fetch<?>.
  This method should throw if the entity types underlying the fetch models are not compatible (one should be a supertype of the other, or vice versa).

Expected outcome

More convenient and type safe use of fetch.unionWith.