api: Add RoutingType to BackendTrafficPolicy

[jukie]

What type of PR is this?

Adds API field to BackendTrafficPolicy so that RoutingType can be configured on a per-route/gateway basis

What this PR does / why we need it:
This will be relevant for ingress-nginx migrations and is needed to support thenginx.ingress.kubernetes.io/service-upstream annotation.

Which issue(s) this PR fixes:

Fixes #7926

Release Notes: No

[netlify]

✅ Deploy Preview for cerulean-figolla-1f9435 canceled.

Name	Link
🔨 Latest commit	9fc7540
🔍 Latest deploy log	https://app.netlify.com/projects/cerulean-figolla-1f9435/deploys/696b0ea6d622200008228c42

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.83%. Comparing base (d0f72f6) to head (9fc7540).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7927   +/-   ##
=======================================
  Coverage   72.82%   72.83%           
=======================================
  Files         237      237           
  Lines       35475    35475           
=======================================
+ Hits        25835    25837    +2     
+ Misses       7801     7799    -2     
  Partials     1839     1839           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[jukie]

/retest