Configure PriorityClassName on PromFed chart

[mallardduck]

Is your feature request related to a problem? Please describe.
Allow setting Priority ClassName on the PromFed Pod

Describe the solution you'd like
Originally submitted via community in: #111
Using new PR based on recent major changes: #195
This tracks 2.11.2 so we should backport to 2.11 branch; backport PR: #252

[mallardduck]

Merged and Tagged an RC here: https://github.com/rancher/prometheus-federator/releases/tag/v3.1.0-rc.1

For QA

Testing is most easily done by downloading the chart from the release (linked above). And installing via helm CLI - unlike Monitoring directly this often works fine when installing outside of the UI.

The only change is an addition of priorityClassName field on the values file. This controls only the priority class of PromFed itself and not the Project Monitoring charts that PromFed deploys. As such for testing, we need to validate that PromFed pod starts up fine (both with and without a priority class set).

[deepakpunia-suse]

Bug Verification Details:

• Rancher: v2.11-head
• RKE2: v1.31.8+rke2r1
• prometheus-federator: 106.1.0+up3.1.0-rc.2

Steps Performed:

1. Installed the monitoring chart.
2. Clicked to install the prometheus-federator chart and selected the option to "Customize Helm options before install".
3. Checked that priorityClassName was set to '' by default (priorityClassName: '').
4. Installed prometheus-federator with default settings and checked that the installation completed successfully without any issues.
5. Uninstalled and then re-installed prometheus-federator with the priorityClass set to rancher-critical (i.e., priorityClassName: rancher-critical).
6. After installation, checked that the prometheus-federator deployment had the specified priorityClassName parameter:

   # kubectl get deploy prometheus-federator -o yaml | grep priorityClassName
         priorityClassName: rancher-critical