Add a flag to configure the default cloud environment

[aramase]

The current logic uses AzurePublicCloud as default option when no cloud name is provided.

secrets-store-csi-driver-provider-azure/pkg/provider/provider.go

Lines 82 to 86 in 6dce6a9

	if cloudName == "" {
	env = azure.PublicCloud
	} else {
	env, err = azure.EnvironmentFromName(cloudName)
	}

Add a flag in the provider to configure default cloud to use if cloudName is not provided in SecretProviderClass.
This will be useful in custom cloud envs for defining AzureStackCloud as default + setting AZURE_ENVIRONMENT_FILEPATH one-time at the provider level instead of every SecretProviderClass.