feat(recommendations): [DIS-802] Add static utm_source to url

[mmiermans]

Goal

Add a utm_source query parameter to the URL to:

1. Allow publishers to attribute traffic coming from NewTab.
2. Attribute Pocket sign-ups to NewTab.

I'd love feedback/perspectives on:

• Whether this is a reasonable short-term solution.

Implementation Decisions

• Kirill requested that the utm_source value is distinct from existing feeds, but it may be the same value for ES, FR, IT. Before we migrate existing markets over to this API, the utm_source value should be unique per ScheduledSurface. DIS-803

Deployment steps

• FOR POCKET DEVELOPERS ONLY - Post in #changelog:

NewTab recommendation urls for France, Italy, Spain now have a utm_source parameter that's set to pocket-newtab-bff, such that publishers can attribute the traffic we send their way, and we can track sign-ups. #51

References

JIRA ticket:

• https://getpocket.atlassian.net/browse/DIS-802?atlOrigin=eyJpIjoiN2QwZjIwZGE3MTkxNDAwNmE3OGQ2NzkzYTEzMTVjYWQiLCJwIjoiamlyYS1zbGFjay1pbnQifQ

Slack thread:

• https://pocket.slack.com/archives/C04LKH4J1M0/p1686088250963949

[jpetto]

lgtm. do we need to handle the case where the url already has a utm_source query param? or is that overly cautious? (i can't remember if corpusItem has that data removed or not...)

[mmiermans]

Good call-out @jpetto. There are 148 items in the Corpus that contain utm_source in the url.

I added a test to ensure utm_source is overridden by the Firefox API proxy.