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/encoder.go

@@ -0,0 +1,51 @@
+package xml
+
+// writer interface used by the xml encoder to write an encoded xml
+// document in a writer.
+type writer interface {
+
+	// Write takes in a byte slice and returns number of bytes written and error
+	Write(p []byte) (n int, err error)
+
+	// WriteRune takes in a rune and returns number of bytes written and error
+	WriteRune(r rune) (n int, err error)
+
+	// WriteString takes in a string and returns number of bytes written and error
+	WriteString(s string) (n int, err error)
+
+	// String method returns a string
+	String() string
+
+	// Bytes return a byte slice.
+	Bytes() []byte
+}
+
+// Encoder is an XML encoder that supports construction of XML values
+// using methods. The encoder takes in a writer and maintains a scratch buffer.
+type Encoder struct {
+	w       writer
+	scratch *[]byte
+}
+
+// NewEncoder returns an XML encoder
+func NewEncoder(w writer) *Encoder {
+	scratch := make([]byte, 64)
+
+	return &Encoder{w: w, scratch: &scratch}
+}
+
+// String returns the string output of the XML encoder
+func (e Encoder) String() string {
+	return e.w.String()
+}
+
+// Bytes returns the []byte slice of the XML encoder
+func (e Encoder) Bytes() []byte {
+	return e.w.Bytes()
+}
+
+// RootElement builds a root element encoding
+// It writes it's start element tag. The value should be closed.
+func (e Encoder) RootElement(element StartElement) Value {
+	return newValue(e.w, e.scratch, element)
+}