Reduce repetition in codebase

[mitchnielsen]

As we build out the provider, we've started seeing opportunities to DRY up the codebase a bit. Let's use this top-level issue to track places we can reduce repetition in the codebase to keep it as consistent and maintainable as possible.

• Client creation in internal/provider/resources/block.go (context)
• HTTP requests in internal/client/* (lots of repetition here when creating the request, setting headers, executing the request, and checking the status code; remember to pass the close function as a result so we can defer the call to it from the caller)
• Diagnostics creation (Use helper functions for consistent Diagnostics #212)
• Enums for resource type name references (context: Improve Diagnostics consistency #213 (comment))
• others?