Add pseudonymization enrichment to Snowplow

What does this MR do and why?

This MR adds the pseudonymization enrichment to Snowplow with a placeholder salt to achieve feature parity with what we have on production since PII enrichment was added in analytics-stack in https://gitlab.com/gitlab-org/analytics-section/product-analytics/analytics-stack/-/merge_requests/85.

Screenshots or screen recordings

The hashed user_ids are after adding the pseudonymization, while 1 is before.

Before & After

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.