[azd] Separate template fetch from init into its own command: generate (proposal)

[robconery]

Currently the init command does two things: creates the azure directory with default settings as well as handling the template logic, which includes:

• evaluating the passed in name
• directory check (if empty)
• fetching and unzipping

I propose we reduce the complexity of init and move the template logic to a new command: generate. This follows what other frameworks do (namely Rails and Express among others). With a dedicated generate command, we could build something more interactive and friendly, including showing the end user what's available for each language/template.