feat: Allow users set upgrade_settings.max_surge property

[mloskot]

Azure Kubernetes Service changed the default max surge
in October release, so that for clusters based
on Kubernetes >1.28 max surge defaults to 10%, see
https://github.com/Azure/AKS/releases/tag/2023-10-01

Previously it was left blank which implied use of
value 1 under the bonnet.

Using the current version of Terraform AzureRM 3.86.0
leads to implicit resetting of the max_surge:

~ default_node_pool {
            name                         = "default"
            ...
            # (23 unchanged attributes hidden)

          - upgrade_settings {
              - max_surge = "10%" -> null
            }
        }

        # (7 unchanged blocks hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.

The only workaround to avoid such confusing annoyance is to
set the max_surge with explicit value e.g. default "10%".
But, this requires max_surge to be exposed to end-users of
this module. See also
hashicorp/terraform-provider-azurerm#24020

Closes #6

[BzSpi]

Thanks @mloskot 👍

[RockyMM]

I'm guessing this PR has to be rebased? Actually, it seems obsolete?

[mloskot]

@RockyMM Yes, it looks like the main commit 55a6808 replaces this PR.