Add config for the header use by the general ingress

[nanassito]

💸 TL;DR

We now have a generic ingress for thrift traffic that routes the traffic based on a header. This means that looking at a service config we don't see what service the traffic is going to and that the application need to set the header manually unlike every other configuration bit.

So this PR adds a new configuration to set this header.

📜 Details

[Design Doc]()

[Jira]()

🧪 Testing Steps / Validation

✅ Checks

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

[nanassito]

Ideally I'd like to add tests but I'm not sure how. I haven't found a way to check the headers yet.