Add ServicePayload to remove nested Service attribute from Service struct

[pdecat]

This PR resolves #57

[stmcallister]

Totally agree with this change. This is how we design resources today. Just one ask before approving. Will you also fix the service_test.go? I received the following error when I ran the tests. Thanks!

==> Checking that code complies with gofmt requirements... ✔
==> Checking that code complies with go vet requirements...# github.com/heimweh/go-pagerduty/pagerduty
vet: pagerduty/service_test.go:41:27: v.Service undefined (type *Service has no field or method Service)
 ✔
==> Checking for misspelling errors... ✔
==> Checking that code complies with staticcheck requirements...pagerduty/service_test.go:41:27: v.Service undefined (type *Service has no field or method Service) (compile)
pagerduty/service_test.go:111:27: v.Service undefined (type *Service has no field or method Service) (compile)
make: *** [staticcheck] Error 1

[heimweh]

Nice job @pdecat! Really appreciate it! Been wanting to tackle this for some time now 😅
Unfortunately, we have more examples of this scattered across the codebase that I'd like to clean up as well.
Let's use this and the new resources as an example going forward and try to align the existing resources the same way 👍

[pdecat]

Fixed the tests:

# make test
==> Checking that code complies with gofmt requirements... ✔
==> Checking that code complies with go vet requirements... ✔
==> Checking for misspelling errors... ✔
==> Checking that code complies with staticcheck requirements... ✔
==> Checking that code complies with golint requirements... ✔
==> Checking that code complies with unit tests...
ok      github.com/heimweh/go-pagerduty/pagerduty       0.048s  coverage: 62.3% of statements

[pdecat]

Submitted #61 to enable Github Actions to run them automatically.

[stmcallister]

Awesome! Thank you!! 🎉