Add --init flag allowing users to scaffold the toml

[thorstenhans]

This PR adds a new --init flag. It allows users to scaffold the spin-pluginify.toml.

If present, user must specify --name and --package as well:

spin pluginify --init --name <PLUGIN-NAME> --package <./PATH/TO/EXECUTABLE>

Help docs for --name and --package are grouped under Init Options:

Init Options:
  -n, --name <NAME>        The name of the plugin (requires --init)
  -p, --package <PACKAGE>  Path to the plugin executable (requires --init)

[itowlson]

Ooh! This looks great! The one change I'd like is to unify your new pluginify manifest struct with the existing pluginify manifest struct. The existing struct is PackagingSettings and it will need to be made Serialise but I think it's worth it. (But if there's a compelling reason to split them then I'm fine with being talked around!)

(The other comment is me musing on restructuring some parts of the CLI and doesn'r require immediate action.)

[itowlson]

Brilliant, thanks Thorsten!