feat: refactor CopyTemplateFiles to support different file system

[howieyuen]

related: #72 (comment)

Refactor CopyTemplateFiles()(rename to RenderLocalTemplate()) to support different file system, not only local directory:

1. For the built-in templates, we must use the go:embed command, otherwise, we cannot compile it into binary, and we cannot access the built-in templates directory because of a relative path.
2. Render the template to the specified directory, the whole process is divided into 3 steps:
   1. ReadTemplate(): read the local path to srcFS
   2. RenderFSTemplate(): render srcFS to destFS
   3. WriteToDisk(): write to disk
3. For third-party integration scenarios, you can choose one:
   1. Local directory: RenderLocalTemplate()
   2. Self-provided memory file system: RenderFSTemplate()
   3. Built-in template that comes with the scaffold package: GetInternalTemplates()

[coveralls]

Coverage decreased (-0.2%) to 73.372% when pulling 1b62f91 on howieyuen:scaffold-servitization into 1c2494f on KusionStack:main.

[elliotxx]

lgtm