Cloudwatch: Fix Unexpected error (#74420)

Fix unexpected error when creating a new cloudwatch datasource.

Involves a fair amount of refactoring, so if this causes unexpected issues related to region fetching we can turn this off with the cloudwatchNewRegionsHandler feature toggle, although we do not predict it will so we are enabling it to default to true and hope to remove it shortly.

pkg/tsdb/cloudwatch/models/api.go

@@ -21,6 +21,7 @@ type RequestContext struct {
 	MetricsClientProvider MetricsClientProvider
 	LogsAPIProvider       CloudWatchLogsAPIProvider
 	OAMAPIProvider        OAMAPIProvider
+	EC2APIProvider        EC2APIProvider
 	Settings              CloudWatchSettings
 	Features              featuremgmt.FeatureToggles
 }
@@ -41,6 +42,10 @@ type AccountsProvider interface {
 	GetAccountsForCurrentUserOrRole() ([]resources.ResourceResponse[resources.Account], error)
 }
 
+type RegionsAPIProvider interface {
+	GetRegions() ([]resources.ResourceResponse[resources.Region], error)
+}
+
 // Clients
 type MetricsClientProvider interface {
 	ListMetricsWithPageLimit(params *cloudwatch.ListMetricsInput) ([]resources.MetricResponse, error)