Add support for DASH ENI Counters

[vivekrnv]

Depends on sonic-net/sonic-swss-common#904

1. Add support for DASH ENI Flex Counter and added a UT
2. Refactored the Flexcounter UT to not use hardcoded oid and instead generate in the test based on object type.

Note: Google Test filter = *FlexCounter*

[==========] Running 7 tests from 1 test suite.
[----------] Global test environment set-up.
[----------] 7 tests from FlexCounter
[ RUN      ] FlexCounter.addRemoveCounter
[       OK ] FlexCounter.addRemoveCounter (16804 ms)
[ RUN      ] FlexCounter.queryCounterCapability
[       OK ] FlexCounter.queryCounterCapability (1050 ms)
[ RUN      ] FlexCounter.noSupportedCounters
[       OK ] FlexCounter.noSupportedCounters (0 ms)
[ RUN      ] FlexCounter.addRemoveCounterPlugin
[       OK ] FlexCounter.addRemoveCounterPlugin (1 ms)
[ RUN      ] FlexCounter.addRemoveCounterForPort
[       OK ] FlexCounter.addRemoveCounterForPort (2000 ms)
[ RUN      ] FlexCounter.bulkCounter
[       OK ] FlexCounter.bulkCounter (12604 ms)
[ RUN      ] FlexCounter.counterIdChange
[       OK ] FlexCounter.counterIdChange (6300 ms)
[----------] 7 tests from FlexCounter (38760 ms total)

[----------] Global test environment tear-down
[==========] 7 tests from 1 test suite ran. (38761 ms total)
[  PASSED ] 7 tests.

[vivekrnv]

/azpw run Azure.sonic-sairedis

[mssonicbld]

/AzurePipelines run Azure.sonic-sairedis

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).