Add template support to `systemd_unit`

Creating from #11402

Describe the Enhancement:

I would like to be able to specify a template (and required configurations) when defining a systemd unit file with systemd_unit.

Describe the Need:

When dealing with complex systemd unit files within a custom resource, it would be useful to be able to expose the ability to override the contents via a template file rather than a string or hash.

In the instance of sous-chefs/tomcat since we are providing a unit file with sensible defaults and expose the ability to override that template to the user, we cannot currently utilize the systemd_unit resource and instead fall back to managing the unit file directly with template and interacting with systemd with execute

https://github.com/sous-chefs/tomcat/pull/358#discussion_r492897903

Current Alternative

As demonstrated in the above cookbook, we are managing the unit file directly with template rather than utilizing the systemd_unit resource.

Can We Help You Implement This?:

This is a reasonable request or does it add too much complexity to the resource?

In terms of implementation, I was thinking something like the following.

Addition of properties such as these added to systemd_unit:

property :template_source, String,
  description: "The location of a template file. By default, #{ChefUtils::Dist::Infra::CLIENT} looks for a template file in the /templates directory of a cookbook."

property :template_cookbook, [String, Hash],
  description: "The cookbook in which a file is located (if it is not located in the current cookbook). The default value is the current cookbook.",
  desired_state: false

property :template_variables, [String, Hash],
  description:  "The variables property of the template resource can be used to reference a partial template file by using a Hash.",
  default: {}

property :template_local, [ TrueClass, FalseClass ],
  default: false, desired_state: false,
  description: "Load a template from a local path. By default, the #{ChefUtils::Dist::Infra::CLIENT} loads templates from a cookbook's /templates directory. When this property is set to true, use the source property to specify the path to a template on the local node."

From there I believe there just needs to be logic within manage_unit_file to detect content usage vs template_source and then use the template resource from there.

https://github.com/chef/chef/blob/master/lib/chef/provider/systemd_unit.rb#L259

Start with lib/chef/provider/systemd_unit.rb at manage_unit_file, then review the proposed template properties and the existing systemd_unit resource behavior. Done means systemd_unit supports template-based unit contents and required template configuration alongside its current content options, with behavior suitable for the tomcat use case.