Support BER long-form length encoding in AARE parsing

[marco92]

Pull request: #115

In the Application Association Response (AARE, aare.py), only ASN.1 BER short-form length encoding was previously supported. This caused parsing to fail for responses using long-form length encoding.

This change adds automatic detection of BER short-form vs long-form length encoding and correctly parses the length in both cases.

Reference (ASN.1 BER length encoding):
https://www.oss.com/asn1/resources/asn1-made-simple/asn1-quick-reference/basic-encoding-rules.html

This improves compatibility with DLMS/COSEM devices that encode AARE APDUs using BER long-form lengths.

Issue observed on "Kamstrup OMNIA® e-meter" and fix tested successfully.

[marco92]

Sample HDLC PDUs:

Public Client (anonymous):

AARQ: 7EA03A00020CC521108D59E6E6006029A109060760857405080101A60A04080000000000000001BE10040E01000000065F1F040020525FFFFFAE2C7E
AARE: 7EA0482100020CC530847FE6E70061820035A109060760857405080101A203020100A305A103020100A40A04084B414D450235CB3ABE10040E0800065F1F040000101D07EB000711677E

GetRequest: 7EA01C00020CC52132D360E6E600C001C0000100022B0108FF02008F087E
GetResponse: 7EA0182100020CC5525F4DE6E700C401C000068D0DC2D71E857E

Management Client (HLS GMAC, challenge length=16)

AARQ: 7EA06E00200CC52D108D9BE6E600605DA109060760857405080103A60A04084B414D450235CB3A8A0207808B0760857405080205AC128010F634FDAAD6E185A3D60D7CF2FA901C13BE230421211F308D0DC2D8F1E2B8410C7EE4D8E1F87A9295F1AB54C5C8068D7101F036866C807A7E
AARE: 7EA08C2D00200CC53038D6E6E70061820079A109060760857405080103A203020100A305A10302010EA40A04084B414D450235CB3A88020780890760857405080205AA2280202511074709B1DC92D6F205BABAFDB42A6B4FC5D3921A58C5444F4CD997E87F00BE230421281F30000009E180385146C45DF12A9179EA7CDDA63F87D0DA8069778EC162147FF5207E