add Terraform wrapper types to generate docs with default value

[maksym-nazarenko]

This PR introduces wrapper types to generate Terraform docs with default value.
It is a drop-in replacement implemented as function.
To enable this feature, just wrap attribute in schema with proper typed wrapper:

"address_pool": defaultaware.StringAttribute(
	schema.StringAttribute{
		Optional:    true,
		Computed:    true,
		Default:     stringdefault.StaticString("static-only"),
		Description: "IP pool, from which to take IP addresses for the clients. If set to static-only, then only the clients that have a static lease (added in lease submenu) will be allowed.",
	},
),

and next run of make generate will generate documentation with Default: .... suffix.

[ddelnano]

Glad to see that there is a way to support this until upstream handles this better. Thanks for figuring this out!