rbd: add aws-sts-metdata encryption type

With Amazon STS and kubernetes cluster is configured with
OIDC identity provider, credentials to access Amazon KMS
can be fetched using oidc-token(serviceaccount token).
Each tenant/namespace needs to create a secret with aws region,
role and CMK ARN.
Ceph-CSI will assume the given role with oidc token and access
aws KMS, with given CMK to encrypt/decrypt DEK which will stored
in the image metdata.

Refer: https://docs.aws.amazon.com/STS/latest/APIReference/welcome.html
Resolves: #2879

Signed-off-by: Rakshith R <rar@redhat.com>

vendor/github.com/aws/smithy-go/encoding/xml/element.go

@@ -0,0 +1,91 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Copied and modified from Go 1.14 stdlib's encoding/xml
+
+package xml
+
+// A Name represents an XML name (Local) annotated
+// with a name space identifier (Space).
+// In tokens returned by Decoder.Token, the Space identifier
+// is given as a canonical URL, not the short prefix used
+// in the document being parsed.
+type Name struct {
+	Space, Local string
+}
+
+// An Attr represents an attribute in an XML element (Name=Value).
+type Attr struct {
+	Name  Name
+	Value string
+}
+
+/*
+NewAttribute returns a pointer to an attribute.
+It takes in a local name aka attribute name, and value
+representing the attribute value.
+*/
+func NewAttribute(local, value string) Attr {
+	return Attr{
+		Name: Name{
+			Local: local,
+		},
+		Value: value,
+	}
+}
+
+/*
+NewNamespaceAttribute returns a pointer to an attribute.
+It takes in a local name aka attribute name, and value
+representing the attribute value.
+
+NewNamespaceAttribute appends `xmlns:` in front of namespace
+prefix.
+
+For creating a name space attribute representing
+`xmlns:prefix="http://example.com`, the breakdown would be:
+local = "prefix"
+value = "http://example.com"
+*/
+func NewNamespaceAttribute(local, value string) Attr {
+	attr := NewAttribute(local, value)
+
+	// default name space identifier
+	attr.Name.Space = "xmlns"
+	return attr
+}
+
+// A StartElement represents an XML start element.
+type StartElement struct {
+	Name Name
+	Attr []Attr
+}
+
+// Copy creates a new copy of StartElement.
+func (e StartElement) Copy() StartElement {
+	attrs := make([]Attr, len(e.Attr))
+	copy(attrs, e.Attr)
+	e.Attr = attrs
+	return e
+}
+
+// End returns the corresponding XML end element.
+func (e StartElement) End() EndElement {
+	return EndElement{e.Name}
+}
+
+// returns true if start element local name is empty
+func (e StartElement) isZero() bool {
+	return len(e.Name.Local) == 0
+}
+
+// An EndElement represents an XML end element.
+type EndElement struct {
+	Name Name
+}
+
+// returns true if end element local name is empty
+func (e EndElement) isZero() bool {
+	return len(e.Name.Local) == 0
+}