[CSGI-1827] Add Standards support

[imjaroiswebdev]

Add support for Standards APIs...

• List Standards - GET: /standards
• Update a standard - PUT: /standards/{id}
• List resources' standards scores - GET: /standards/scores/{resource_type}
• List a resource's standards scores - GET: /standards/scores/{resource_type}/{id}

Test cases added...

$ go test -count=1 -v -run TestStandard
=== RUN   TestStandard_List
--- PASS: TestStandard_List (0.00s)
=== RUN   TestStandard_Update
--- PASS: TestStandard_Update (0.00s)
=== RUN   TestStandard_ListTechniServiceStandardScores
--- PASS: TestStandard_ListTechniServiceStandardScores (0.00s)
=== RUN   TestStandard_ListManyTechniServiceStandardScores
--- PASS: TestStandard_ListManyTechniServiceStandardScores (0.00s)
PASS
ok      github.com/PagerDuty/go-pagerduty       0.709s