[6.1.x] Configure pod subnet size through cluster configuration resource

[bernardjkim]

Description

These changes allow the pod subnet size to be configured through the cluster configuration resource.

// config.yaml
kind: ClusterConfiguration
version: v1
spec:
  global:
    podSubnetSize: "26"
...

Installing the Gravity cluster with gravity install --config config.yaml will install the cluster with a pod subnet size of 26. The pod subnet size can also be updated after installation using gravity resource create config.yaml

Type of change

• New feature (non-breaking change which adds functionality)
• This change has a user-facing impact

Linked tickets and other PRs

• Ports Configure pod subnet size through cluster configuration resource #2297

• Requires [6.1.x] Add pod-subnet-size flag planet#788

TODOs

• Self-review the change
• Perform manual testing
• Address review feedback

Testing done

• Verify pod subnet size is set when installing the cluster with --config config.yaml
• Verify pod subnet size is updated when updating the cluster configuration resource