Add support for specifying optional network_security_group_name in Packer Azure ARM configuration

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#### Description

Add support for specifying optional `network_security_group_name` in the Packer Azure ARM configuration. This enhancement will allow users to specify these components if they have a pre-existing Network Security Group in their Azure environment.

#### Use Case(s)

- Allow an already created Azure Network Security Group (`network_security_group_name`) in the user’s Azure Cloud to be used by the temporary Network Interface Packer will create.

#### Potential configuration

Add the following fields to the `Config` struct in `config.go`:
```
type Config struct {
// Existing fields...

// Name of the network security group to use.
NetworkSecurityGroupName string `mapstructure:"network_security_group_name" required:"false"`
}
```

#### Potential References

- Update [template-factory.go](https://github.com/hashicorp/packer-plugin-azure/blob/main/builder/azure/arm/template_factory.go#L132) and relevant functions to incorporate the new field `network_security_group_name`. This may involve modifying how the ARM template is generated and ensuring that these values are correctly referenced and used during the deployment process.
- Update [resource_resolver.go](https://github.com/hashicorp/packer-plugin-azure/blob/main/builder/azure/arm/resource_resolver.go#L36) and relevant functions to validate that the provided `network_security_group_name` exists in the provided Azure Cloud.

Start with the Config struct in config.go, then read the referenced paths in template-factory.go and resource_resolver.go to trace how ARM templates and Azure resources are handled. Done means an optional network_security_group_name can be configured, used for the temporary network interface, and validated against the provided Azure environment.

Written by the indexing model from the issue text.