Add click tracking to in product marketing email campaigns

What does this MR do?

This adds click tracking to the in product marketing email campaigns.

Tracking is done by sending a self describing event to the Snowplow data warehouse. The event uses this schema: iglu:com.gitlab/email_campaigns/jsonschema/1-0-0

Note: Since these changes are behind a feature flag (experiment toggle), we don't include a changelog file here.

Related to:

• Issue: #296133 (closed)
• Epic: gitlab-org/growth&62