feat: add support for priority classes

[sym-stiller]

Description

These changes enable users of the Tyk Helm charts to specify a priorityClassName for each component. By default, no priority class is assigned to the components.

Related Issue

#411

Motivation and Context

When no priority class is specified, Kubernetes will assign the lowest possible priority to pods. These low-priority pods may be preempted by higher-priority pods, which is not ideal for an API gateway.

Test Coverage For This Change

I have templated the charts locally like this:

helm dependency build # only necessary for the umbrella charts, not the component charts
helm template tyk ./ -n tyk --debug

Types of changes

• 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)
• Refactoring or add test (improvements in base code or adds test coverage to functionality)
• Documentation updates or improvements.

Checklist

• Make sure you are requesting to pull a topic/feature/bugfix branch (right side). If PRing from your fork, don't come from your master!
• Make sure you are making a pull request against our master branch (left side). Also, it would be best if you started your change off our latest master.
• My change requires a change to the documentation.
  • I have manually updated the README(s)/documentation accordingly.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.