Add DefaultFunc / Default field for provider configuration

[BBBmau]

When working on the integration for helm, I found that plugin-framework does not support default values within provider configurations.

This is a requirement as helm uses Default values that are often obtained from Environment variables. An example of how this is implemented in SDKv2 can be seen below.

// Provider returns the provider schema to Terraform.
func Provider() *schema.Provider {
	p := &schema.Provider{
		Schema: map[string]*schema.Schema{
			"debug": {
				Type:        schema.TypeBool,
				Optional:    true,
				Description: "Debug indicates whether or not Helm is running in Debug mode.",
				DefaultFunc: schema.EnvDefaultFunc("HELM_DEBUG", false),
			},
			"plugins_path": {
				Type:        schema.TypeString,
				Optional:    true,
				Description: "The path to the helm plugins directory",
				DefaultFunc: schema.EnvDefaultFunc("HELM_PLUGINS", helmpath.DataPath("plugins")),
			},
			"registry_config_path": {
				Type:        schema.TypeString,
				Optional:    true,
				Description: "The path to the registry config file",
				DefaultFunc: schema.EnvDefaultFunc("HELM_REGISTRY_CONFIG", helmpath.ConfigPath("registry.json")),
			},
...

Currently I am unable to complete the provider configuration for helm without the fields.

Proposal

It seems like the same logic from other attributes can be applied for the Provider to contain this field also. I am also assuming that theirs already an ability to grab Environment variables that is similar to that found in SDKv2

References

https://github.com/hashicorp/terraform-plugin-framework/blob/main/provider/schema/schema.go
https://github.com/hashicorp/terraform-plugin-framework/blob/main/resource/schema/string_attribute.go

[BBBmau]

Closing this issue since its a duplicate of #539

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