AB#2299 License check in CLI during init (#366)

* license server interaction
* logic to read from license file
* print license information during init
Signed-off-by: Fabian Kammel <fk@edgeless.systems>
Co-authored-by: Moritz Eckert <m1gh7ym0@gmail.com>

internal/license/file.go

@@ -0,0 +1,28 @@
+package license
+
+import (
+	"encoding/base64"
+	"fmt"
+
+	"github.com/edgelesssys/constellation/internal/file"
+)
+
+func FromFile(fileHandler file.Handler, path string) (string, error) {
+	readBytes, err := fileHandler.Read(path)
+	if err != nil {
+		return "", fmt.Errorf("unable to read from '%s': %w", path, err)
+	}
+
+	maxSize := base64.StdEncoding.DecodedLen(len(readBytes))
+	decodedLicense := make([]byte, maxSize)
+	n, err := base64.StdEncoding.Decode(decodedLicense, readBytes)
+	if err != nil {
+		return "", fmt.Errorf("unable to base64 decode license file: %w", err)
+	}
+	if n != 36 { // length of UUID
+		return "", fmt.Errorf("license file corrupt: wrong length")
+	}
+	decodedLicense = decodedLicense[:n]
+
+	return string(decodedLicense), nil
+}