feat: use cargo generate instead of hardcoding examples source code (…

…#888)

* feat: use `cargo generate` instead of hardcoding examples source code

* feat: use `cargo-generate` as a library

* feat: add `custom/none` example

* use project name `.as_ref()`

* fix: apparently we need to use `auto_path` instead of `subfolder`.

* chore: update `examples` git submodule

* manually set the crate name

* fix: clippy lints

* fix: rocket sub path

* move `use` to top of file

* remove the Shuttle.toml file, if it exists

* feat: create a .gitignore file when copying an example

closes #867

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Co-authored-by: Iulian Barbu <14218860+iulianbarbu@users.noreply.github.com>