Allow for overriding the default Gen instance used for a type in zio-schema-zio-test

[beem812]

Current behavior:
Gen instances for a string are automatically set to Gen.anyString

Problem:
Gen.anyString can produce a character that postgres doesn't like, \u0000, which causes tests against a postgres db to randomly fail and do so extremely rarely making it harder to debug the cause. The use of refined types would also be a use case for this, for instance something with a fairly specific string refinement is better served by starting with a string that closely matches the refinement rather than hoping you eventually get the right kind of string from Gen.anyString.

Solution:
Some way to implicitly override the string gen for anything else, or a way to bake in the generator I want into a schema that can be used in place of the string primitive schema. An annotation might be a good solution.