Use Record to replace PrismaUnion

[medz]

In the current Prisma Dart, because of the problem of multi-type input, PrismaUnion is built. It usually represents two or types:

// String | int
PrismaUnion<String, int>

Create PrismaUnion.$1 or PrismaUnion.$2 when using, but there are problems with specific types and serialization.

After replacement:

// String | int
(String, int)

Explicitly indicate the type, use ('a', null) or (null, 1)`