Prometheus: Fix Azure authentication support (#44407)

Re-adding back Azure authentication support to Prometheus datasource 
after the datasource query logic was rewritten from plugin.json routes to 
Go backend.

Ref #35857

pkg/util/maputil/maputil.go

@@ -0,0 +1,73 @@
+package maputil
+
+import "fmt"
+
+func GetMap(obj map[string]interface{}, key string) (map[string]interface{}, error) {
+	if untypedValue, ok := obj[key]; ok {
+		if value, ok := untypedValue.(map[string]interface{}); ok {
+			return value, nil
+		} else {
+			err := fmt.Errorf("the field '%s' should be an object", key)
+			return nil, err
+		}
+	} else {
+		err := fmt.Errorf("the field '%s' should be set", key)
+		return nil, err
+	}
+}
+
+func GetBool(obj map[string]interface{}, key string) (bool, error) {
+	if untypedValue, ok := obj[key]; ok {
+		if value, ok := untypedValue.(bool); ok {
+			return value, nil
+		} else {
+			err := fmt.Errorf("the field '%s' should be a bool", key)
+			return false, err
+		}
+	} else {
+		err := fmt.Errorf("the field '%s' should be set", key)
+		return false, err
+	}
+}
+
+func GetBoolOptional(obj map[string]interface{}, key string) (bool, error) {
+	if untypedValue, ok := obj[key]; ok {
+		if value, ok := untypedValue.(bool); ok {
+			return value, nil
+		} else {
+			err := fmt.Errorf("the field '%s' should be a bool", key)
+			return false, err
+		}
+	} else {
+		// Value optional, not error
+		return false, nil
+	}
+}
+
+func GetString(obj map[string]interface{}, key string) (string, error) {
+	if untypedValue, ok := obj[key]; ok {
+		if value, ok := untypedValue.(string); ok {
+			return value, nil
+		} else {
+			err := fmt.Errorf("the field '%s' should be a string", key)
+			return "", err
+		}
+	} else {
+		err := fmt.Errorf("the field '%s' should be set", key)
+		return "", err
+	}
+}
+
+func GetStringOptional(obj map[string]interface{}, key string) (string, error) {
+	if untypedValue, ok := obj[key]; ok {
+		if value, ok := untypedValue.(string); ok {
+			return value, nil
+		} else {
+			err := fmt.Errorf("the field '%s' should be a string", key)
+			return "", err
+		}
+	} else {
+		// Value optional, not error
+		return "", nil
+	}
+}