TLV: Add DGI encoding of "GP Scripting Language Annex B"

The DGI encoding is specified in Annex B of the
"GlobalPlatform Systems Scripting Language Specification v1.1.0"

which is an "archived" specification that is no longer published
by GlobalPlatform, despite it being referenced from the GlobalPlatform
Card Specification v2.3, which is the basis of the GSMA eSIM
specifications.

For some reason it was the belief of the specification authors that
yet another format of TLV encoding is needed, in addition to the BER-TLV
and COMPREHENSION-TLV used by the very same specifications.

The encoding of the tag is not really specified anywhere, but I've only
seen 16-bit examples.  The encoding of the length is specified and
implemented here accordingly.

Change-Id: Ie29ab7eb39f3165f3d695fcc1f02051338095697

pySim/utils.py

@@ -358,6 +358,42 @@ def bertlv_parse_one(binary: bytes) -> Tuple[dict, int, bytes, bytes]:
     return (tagdict, length, value, remainder)
 
 
+def dgi_parse_tag_raw(binary: bytes) -> Tuple[int, bytes]:
+    # In absence of any clear spec guidance we assume it's always 16 bit
+    return int.from_bytes(binary[:2], 'big'), binary[2:]
+
+def dgi_encode_tag(t: int) -> bytes:
+    return t.to_bytes(2, 'big')
+
+def dgi_encode_len(length: int) -> bytes:
+    """Encode a single Length value according to GlobalPlatform Systems Scripting Language
+    Specification v1.1.0 Annex B.
+    Args:
+            length : length value to be encoded
+    Returns:
+            binary output data of encoded length field
+    """
+    if length < 255:
+        return length.to_bytes(1, 'big')
+    elif length <= 0xffff:
+        return b'\xff' + length.to_bytes(2, 'big')
+    else:
+        raise ValueError("Length > 32bits not supported")
+
+def dgi_parse_len(binary: bytes) -> Tuple[int, bytes]:
+    """Parse a single Length value according to  GlobalPlatform Systems Scripting Language
+    Specification v1.1.0 Annex B.
+    Args:
+            binary : binary input data of BER-TLV length field
+    Returns:
+            Tuple of (length, remainder)
+    """
+    if binary[0] == 255:
+        assert len(binary) >= 3
+        return ((binary[1] << 8) | binary[2]), binary[3:]
+    else:
+        return binary[0], binary[1:]
+
 # IMSI encoded format:
 # For IMSI 0123456789ABCDE:
 #