refactor: cleaning up load balancers & adding validation to Load Balancer ID's & at import time

[tombuildsstuff]

This PR refactors the Load Balancer resources to use common ID parsers - as a first step to refactoring these resources

It also adds both import-time and field validation for the Load Balancer ID's, rather than just taking a generic Resource ID - so we can ensure these are consistent. It's worth noting this is intentionally a first-step - there's more to do here, but this is a good cut point for now

Tests pass:

[jackofallops]

Thanks @tombuildsstuff - other than a few missed error message tweaks LGTM 👍

[ghost]

This has been released in version 2.24.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "azurerm" {
    version = "~> 2.24.0"
}
# ... other configuration ...

[ghost]

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