fix(storage): Storage Assume Role Support

This commit introduces fixes to enable operators to use AWS dynamic
credentials on the storage side of this plugin, namely validation,
improved credential lifecycle management to allow for storage buckets to
be updated from static to dynamic credentials and vice versa, and a new
way to determine credential types.

It also introduces new tests for dynamic credential usage for the
various functions.

testing/testing.go

@@ -16,7 +16,7 @@ import (
 	"github.com/aws/smithy-go"
 	"github.com/hashicorp/boundary-plugin-aws/internal/credential"
 	"github.com/hashicorp/boundary-plugin-aws/internal/values"
-	awsutilv2 "github.com/hashicorp/go-secure-stdlib/awsutil"
+	awsutilv2 "github.com/hashicorp/go-secure-stdlib/awsutil/v2"
 	tfjson "github.com/hashicorp/terraform-json"
 	"github.com/stretchr/testify/require"
 	"google.golang.org/protobuf/types/known/structpb"