feat: assume role provider credentials (#28)

* feat: assume role provider credentials

This commit updates the awsutil dependency to its latest version, which
supports the capability of using dynamic credentials. The storage service
was refactored to make static credentials optional. The credentials pkg
was refactored to support several attribute fields used for the assume
role provider. The host service still enforces the use of static
credentials.

internal/values/proto.go

@@ -84,6 +84,33 @@ func GetTimeValue(in *structpb.Struct, k string) (time.Time, error) {
 	return t, nil
 }
 
+// GetMapStringString returns a map[string]string value and no error if the given key
+// is found in the provided proto struct input. An error is returned if the key
+// is not found or the value type is not map[string]string.
+func GetMapStringString(in *structpb.Struct, k string, required bool) (map[string]string, error) {
+	mv := in.GetFields()
+	v, ok := mv[k]
+	if !ok {
+		if required {
+			return nil, fmt.Errorf("missing required value %q", k)
+		}
+		return nil, nil
+	}
+	result := map[string]string{}
+	m, ok := v.AsInterface().(map[string]any)
+	if !ok {
+		return nil, fmt.Errorf("unexpected type for value %q: want map[string]string, got %T", k, v.AsInterface())
+	}
+	for mapKey, mapValue := range m {
+		e, ok := mapValue.(string)
+		if !ok {
+			return nil, fmt.Errorf("unexpected type for value in map[%q]: want string, got %T", mapKey, mapValue)
+		}
+		result[mapKey] = e
+	}
+	return result, nil
+}
+
 // StructFields returns a map[string]struct{} of the
 // proto struct input.
 func StructFields(s *structpb.Struct) map[string]struct{} {