Improve support for bit strings in SNMP

The inconvenience of working with bit strings is still not addressed after ZBXNEXT-4428. Zabbix should allow easy conversion from bit strings returned as a sequence of Hex characters, e.g. "1A 2B 3C 4D". Bit strings are recorded from smaller bit numbers to larger, which means that Hexadecimal to decimal preprocessing can not be used here.

This can be done by adding a new conversion format parameter to SNMP walk to JSON preprocessing rule. It's understood that bit strings can be very long in theory yet in practice the limit of 8 bytes for this conversion should not be a significant problem.