Feature: Add PostHog backend tracker

Issue Link: #821 (closed)

What does this MR do

Upstream use Snowplow as tracking service. JH use self-managed PostHog as tracking service.

The purpose of this MR is to add PostHog backend support.

JH SaaS only.

How to test

event list

• https://gitlab.com/gitlab-org/gitlab/-/tree/master/config/events
• https://gitlab.com/gitlab-org/gitlab/-/tree/master/ee/config/events

Visit GitLab like a normal user, go to the PostHog dashboard to view live events data after triggering events.

Reference

Research Doc: https://docs.google.com/document/u/1/d/1-zxCFJLBaul4HXW5NKyCcDr8cCKl7mWo/edit

PostHog Ruby Gem Doc: https://posthog.com/docs/integrate/server/ruby

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cc EM @prajnamas

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This MR involve a bug about Gemfile, fixed in !646 (merged)