Added snmpwalk convert support for Opaque extra types UInt64 and Int64

Added convertion support for additional Opaque types with integers (only which I have in snmpwalks). Ie:

.1.3.6.1.4.1.2021.10.1.6.1 = Opaque: Float: 0.340820
.1.3.6.1.4.1.2021.10.1.6.2 = Opaque: UInt64: 18446744073709551614
.1.3.6.1.4.1.2021.10.1.6.3 = Opaque: Int64: 9223372036854775806
.1.3.6.1.4.1.2021.10.1.6.4 = Opaque: Int64: -2

Notes:

int to snmp hex convertion probably should be in pyasn1 library or pyasn1.codec.ber should know how convert int to ber serialization
really need universally way with BER serialization for any possible subtypes (also possible double)
information how detect Opaque subtype tags I founded there https://github.com/delimitry/snmp-server/blob/master/snmp-server.py

Currently I just added hard fix for self snmpwalks received from user.

etingof / snmpsim

Added snmpwalk convert support for Opaque extra types UInt64 and Int64 #141