Add track options for join a project

What does this MR do and why?

Add structured event tracking for "join a project" selection in the SaaS registration flow

How to set up and validate locally

• Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
• Start registration. On welcome page select Join a project.
• Fill the rest of the form, submit.
• Make sure correct structured event is populated.

To test snowplow events configure Snowplow Micro

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #427472 (closed)