Adds Service Level Objective (SLO) support (#263)

* add SLO support

* fix docs

* fix docs

* small tweaks to formatting

* PR review fixes

* add clarifying sentence

* fix test

* fix provider to Optional fields

make syntax backwards compatible

suppress *_display changes

* because of https://github.com/hashicorp/terraform/issues/6215/ move from TypeMap to TypeList and validate

* add validation to query options

* Revert "add validation to query options"

This reverts commit 6338ed2.

* fix docs

address possible difference in state value for thresholds

* apply same logic to query since it is now TypeList

* fix

* ValueType casts as []interface{} for TypeList

* fix dangling character

datadog/provider.go

@@ -44,6 +44,7 @@ func Provider() terraform.ResourceProvider {
 			"datadog_integration_aws":                      resourceDatadogIntegrationAws(),
 			"datadog_integration_pagerduty":                resourceDatadogIntegrationPagerduty(),
 			"datadog_integration_pagerduty_service_object": resourceDatadogIntegrationPagerdutySO(),
+			"datadog_service_level_objective":              resourceDatadogServiceLevelObjective(),
 		},
 
 		ConfigureFunc: providerConfigure,