Add option to use refer in channel classification if mkt is null

[rlh1994]

Description

FOR DISCUSSION

This PR adds a var to enable the use of refr fields in place of mkt fields when they are null in the default channel group query. This is off by default to make this a non-breaking change (and it is non-GA type behaviour. This has been mentioned a few times and done a few times by customers (see here).

What type of PR is this? (check all applicable)

• [x] 🍕 Feature
• [ ] 🐛 Bug Fix
• [ ] 📝 Documentation Update
• [ ] 🎨 Style
• [ ] 🧑‍💻 Code Refactor
• [ ] 🔥 Performance Improvements
• [ ] ✅ Test
• [ ] 🤖 Build
• [ ] 🔁 CI
• [ ] 📦 Chore (Release)
• [ ] ⏩ Revert

Related Tickets & Documents

Checklist

• [ ] ❗️ I have verified that these changes work on Redshift
• [ ] 💣 Is your change a breaking change?
• [x] 📖 I have updated the CHANGELOG.md

Added tests?

• [ ] 👍 yes
• [x] 🙅 no, because they aren't needed
• [ ] 🙋 no, because I need help

Added to documentation?

• [ ] 📓 internal package docs (ymls, macros, readme, if applicable)
• [ ] 📕 I have raised a Snowplow documentation PR if applicable (Link here if required)
• [ ] 🙅 no documentation needed

[miike]

I think we probably just need something in the docs to the effect that this only impacts medium and source (in terms of coalescing from referrer) and not other UTM fields (campaign, content etc). I'm not sure if people expect / want those fields coalesced as well.

[rlh1994]

I think we probably just need something in the docs to the effect that this only impacts medium and source

Yeah we'll add details to the var for this and probably flesh out the docs on this more generally. From what I could tell refr doesn't have a campaign equivalent, and we don't use any other fields (e.g. content)