OAM-8189 Add BucketingKey, BucketingValue to ExperimentEvent

[julieahnt]

Closes #

💸 TL;DR

Add general field for bucketing key and value instead of adding new rand unit directly as in device_id. Instead, we can use the new fields for any new rand units.
e.g.

• BucketingKey: "post_id"
• BucketingValue: "1k3xqmf"

📜 Details

Jira OAM-8189

🧪 Testing Steps / Validation

✅ Checks

• CI tests (if present) are passing
• Adheres to code style for repo
• Contributor License Agreement (CLA) completed if not a Reddit employee

[kylelemons]

A few quick questions:

• Have you shared more details about this change in #baseplate-go? There's not a lot to go on here.
• Have you sent a companion PR for the v2 version of this code?
• Do we even need this in baseplate.go v0, or can we put this in the v2 version only?

[julieahnt]

Hi @kylelemons, thank you for the comment.

The PR was to add a new field to the event which we will be only used on GQL. I don't think there is any equivalent code in V2, However, I am also exploring to migrate GQL to use GRPC event collector, and that is why I mark this PR as draft.