support switch to decide whether generate tagless key when writing sst file

[Nicole00]

add a switch to decide whether write vertex without tag for sst import mode.

if enable tagless, write vertex key without tag and with empty value, write vertex key with tag and real value.
if not enable tagless, just write vertex key with tag and real value.

default config value for enableTagless will be false.

related issue and pr: vesoft-inc/nebula#4457 vesoft-inc/nebula#4727