Add distinct_id to group_identify

[skoob13]

Our Node.js SDK allows users to set the distinct_id property in the group_identify method. However, this SDK doesn't have the same functionality. This leads to group_identify events made by "artificial users," confusing customers.

Example:

[darkopia]

+1 https://posthoghelp.zendesk.com/agent/tickets/22530
+1 https://posthoghelp.zendesk.com/agent/tickets/21254

[dmarticus]

fixed with the above PR from @sibinms