Add datalayer events for welcome trial flow

What does this MR do and why?

Related to gitlab-com/marketing/digital-experience/buyer-experience#990 (moved) Relevant for SaaS /users/sign_up/welcome/continuous_onboarding_getting_started and /users/sign_up/welcome/

How to set up and validate locally

1. Create a Google Tag Manager account and container ID. You should just need to follow the step 1 of the instructions. The GTM container can be empty - it just needs to exist.
2. In gitlab.yml, add a your GTM ID in the extra block for both the google_tag_manager_id and google_tag_manager_nonce_id values. Make sure to restart GDK (or do this step prior to starting GDK) for settings to take effect.

   extra:
    google_tag_manager_id: 'GTM-YOURIDHERE'
    google_tag_manager_nonce_id: 'GTM-YOURIDHERE'

3. Set your GDK instance to act as SaaS
4. Start or restart your GDK
5. bundle exec rails -c to open the Rails console and enable some feature flags:
6. Feature.enable(:gitlab_gtm_datalayer)
7. Open the Google Tag Assistant
8. Visit http://localhost:3000/users/sign_up/welcome/ and http://localhost:3000/users/sign_up/welcome/continuous_onboarding_getting_started from the Tag Assistant

How to reach the views

Free trial flow: https://docs.google.com/presentation/d/1NjpeT5Y4MFXxv1sDq2O3CiUWAtbttjib6Wbwg9bpWTs/edit#slide=id.g163710a9650_0_0

Here are what the prospective views look like

/users/sign_up/welcome/continuous_onboarding_getting_started	/users/sign_up/welcome/
Go to /users/sign_up/groups_projects/new, then "create project" with a project/group, and click on "skip trial"	Go to /users/sign_up/welcome/

If the paths aren't available, you will need to reach them with a non-admin local dummy account.

Describe in detail what your merge request does and why.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.