load_balancing: support least_outstanding_requests steering policy

[tc80]

Description

Support the new least_outstanding_requests steering policy.
This cannot be merged until the feature is released and API docs are updated.
Edit: the API docs are updated now, so this has been taken out of draft

Has your change been tested?

It is a documentation-only change.

What sort of change does your code introduce/modify?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented in cloudflare/api-schemas
  and relies on stable APIs.

[github-actions]

changelog detected ✅

[tc80]

The API docs are now live, and this can now be reviewed/merged. 👍

https://developers.cloudflare.com/api/operations/load-balancers-create-load-balancer
https://developers.cloudflare.com/api/operations/load-balancer-pools-create-pool

[github-actions]

This functionality has been released in v0.70.0.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!