Extract Go Client SDK into separate module

[jawnsy]

The Go Client SDK included in this provider is not Terraform-specific, and we can refactor it into a separate Go module for use in other applications. We could then also simplify much of this code by using code generation, related to #13

[mitchnielsen]

@parkedwards - is this already complete given that we have:

https://github.com/PrefectHQ/terraform-provider-prefect/tree/main/internal/client

Or is Go Client SDK referring to something else here?