Use a templating engine for Spin templates

[radu-matei]

The current implementation for templates is rather simple:

❯ spin templates add --help
spin-templates-add 0.1.0
Add a template repository locally

USAGE:
    spin templates add [OPTIONS] --name <name>

OPTIONS:
        --branch <branch>    The optional branch of the git repository
        --git <git>          The URL of the templates git repository. The templates must be in a git repository in a
                             "templates" directory
        --local <local>      Local directory to add as a template
        --name <name>        The name of the templates repository

Templates can added from:

• a git repository with a templates/ directory in the root, containing individual templates as directories
• a single local directory

While they are named "templates", there is no templating engine configured, and creating a new application based on a template using spin new will simply copy the contents of the selected "template".
There are a few options for selecting a templating engine - for example, yo-wasm uses EJS, while cargo generate uses the Liquid template language.

(One very interesting approach of cargo generate is allowing template-defined placeholders, which allow for template authors to define custom variables and either get their values through user interaction, or use a supplied default value.)

Ideally, we would pick a templating language and implement support for it with Spin templates.