Automatic merge from submit-queue (batch tested with PRs 57926, 57930). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Add proxy_read_timeout flag to kubeapi_load_balancer charm.

**What this PR does / why we need it**: Add proxy_read_timeout flag to kubeapi_load_balancer charm.

**Release note**:
```release-note
Add proxy_read_timeout flag to kubeapi_load_balancer charm.
```