attestation: add Azure TDX attestation (#2827)

* Implement Azure TDX attestation primitives
* Add default measurements and claims for Azure TDX
* Enable Constellation on Azure TDX

---------

Signed-off-by: Daniel Weiße <dw@edgeless.systems>

internal/encoding/encoding.go

@@ -0,0 +1,71 @@
+/*
+Copyright (c) Edgeless Systems GmbH
+
+SPDX-License-Identifier: AGPL-3.0-only
+*/
+
+// Package encoding provides data types and functions for JSON or YAML encoding/decoding.
+package encoding
+
+import (
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+)
+
+// HexBytes is a byte slice that is marshalled to and from a hex string.
+type HexBytes []byte
+
+// String returns the hex encoded string representation of the byte slice.
+func (h HexBytes) String() string {
+	return hex.EncodeToString(h)
+}
+
+// UnmarshalJSON implements the json.Unmarshaler interface.
+func (h *HexBytes) UnmarshalJSON(data []byte) error {
+	var hexString string
+	if err := json.Unmarshal(data, &hexString); err != nil {
+		return err
+	}
+	// special case to stay consistent with yaml unmarshaler:
+	// on empty string, unmarshal to nil
+	if hexString == "" {
+		*h = nil
+		return nil
+	}
+	return h.unmarshal(hexString)
+}
+
+// MarshalJSON implements the json.Marshaler interface.
+func (h HexBytes) MarshalJSON() ([]byte, error) {
+	return json.Marshal(h.String())
+}
+
+// UnmarshalYAML implements the yaml.Unmarshaler interface.
+func (h *HexBytes) UnmarshalYAML(unmarshal func(any) error) error {
+	var hexString string
+	if err := unmarshal(&hexString); err != nil {
+		// compatibility mode for old state file format:
+		// fall back to unmarshalling as a byte slice for backwards compatibility
+		var oldHexBytes []byte
+		if err := unmarshal(&oldHexBytes); err != nil {
+			return fmt.Errorf("unmarshalling hex bytes: %w", err)
+		}
+		hexString = hex.EncodeToString(oldHexBytes)
+	}
+	return h.unmarshal(hexString)
+}
+
+// MarshalYAML implements the yaml.Marshaler interface.
+func (h HexBytes) MarshalYAML() (any, error) {
+	return h.String(), nil
+}
+
+func (h *HexBytes) unmarshal(hexString string) error {
+	bytes, err := hex.DecodeString(hexString)
+	if err != nil {
+		return fmt.Errorf("decoding hex bytes: %w", err)
+	}
+	*h = bytes
+	return nil
+}