[submodule]: Advance sonic-snmpagent. (#5213)

Update sonic-snmpagent submodule to include below commits:
1a2b62a [Namespace]: Fix SAI_ID key used in cpfcIfTable and csqIfQosGroupStatsTable implementation (#138)
d06f00c [pytest/coverage]: add coverage support (#156)
90e9f2e [Namespace]: Simplify sync_d functions to use higher order (#154)
b5815d9 [LLDP]: Modify OID index of LLDPRemTableUpdater MIB (#155)
d5f2b92 [Multiasic]: Provide namespace support for ipNetToMediaPhysAddress (#129)
166c221 [Namespace]: Fix interface counters in RFC 1213 (#145)

Signed-off-by: SuvarnaMeenakshi <sumeenak@microsoft.com>