Handle the creation of default scaling policies

[mszostok]

Description

This PR allows to manage default scaling policies without importing them. If there is create operation against the default scaling policy, it's changed to update.

Without this PR, the following error is thrown:

   ╷
   │ Error: expected status code 200, received: status=409 body={"message":"scaling policy with the same name already exists", "fieldViolations":[]}
   │
   │   with castai_workload_scaling_policy.default["staging"],
   │   on import.tf line 8, in resource "castai_workload_scaling_policy" "default":
   │    8: resource "castai_workload_scaling_policy" "default" {
   ╵

I also update the import section to describe easier way of doing that.

Limitations

1. Default policies are created in the context of a given cluster ID. As a result, you need to collect and provide all the cluster IDs where you want to change the default policy.
2. Most of the props are required anyway so if they want to update only a few props, they are still required to provide all of them.

[mszostok]

bumping the SDK causes tests to fail, I will report that to the relevant teams.

[tech-writer-rob]

Suggested change

	For each connected cluster, a default scaling policy is created. An existing scaling policy can be imported into the
	Terraform state using the `terraform import` command or the [`import`](https://developer.hashicorp.com/terraform/language/import#syntax) block.
	For each connected cluster, a default scaling policy is created. An existing scaling policy can be imported into the
	Terraform state using the `terraform import` command or the [`import`](https://developer.hashicorp.com/terraform/language/import#syntax) block (recommended for Terraform 1.5.0+).

[tech-writer-rob]

Suggested change

	3. Review the `generated.tf` file and ensure that the imported scaling policy is correct. Terraform sets zero values,
	e.g., for `look_back_period_seconds`. All such properties can be removed to use the defaults.
	3. Review the `generated.tf` file and ensure the imported scaling policy is correct. Terraform will generate this file by setting values equal to zero for certain configuration parameters.
	For example:
	```hcl
	cpu {
	look_back_period_seconds = 0
	}

You can simplify the configuration by:

• Removing fields set to 0 to use default values
• Keeping only the fields you want to configure explicitly

[tech-writer-rob]

The zero values were a little unclear to me.

I have yet to figure out how to include a code block in a suggestion without breaking the suggestion syntax...

[tech-writer-rob]

Suggested change

	### Import using `terraform import` command
	### Import using the `terraform import` command

[tech-writer-rob]

Suggested change

	## Upsert scaling policy
	The recommended way is to [import](#importing) the scaling policy and then apply the changes to the policy.
	However, if that’s not possible, you can define the default policy resource yourself. The CAST AI Terraform provider,
	will update the existing policy instead of returning an error.
	## Upsert scaling policy
	The recommended way is to [import](#importing) the scaling policy and then apply the changes to the policy.
	However, if that’s not possible, you can define the default policy resource yourself. The CAST AI Terraform provider
	will update the existing policy instead of returning an error.