Add crate for serving Scalar via utoipa

[junlarsen]

Hi, thanks for your excellent work on utoipa!

This is a PR for adding Scalar API visualization to the utoipa ecosystem. Scalar is yet another web application for OpenAPI specifications, but I prefer it over Redoc/Rapidoc since it shows the model types too! (also, it's very pretty :D)

Most of the things here were mimicked after the Redoc PR (#720).

• The crate has been added to the CI/publish/test scripts
• The todo-axum/todo-actix/todo-rocket examples now serve Scalar too under /scalar
• The crate has been implemented for axum, actix-web, and rocket

Here are some things that I didn't implement now, but could definitely see being useful:

• Configuration
• Themes & Styling

And finally, here is a preview of what the todo-axum docs look like :smiley:

[newtykip]

Scalar is gorgeous - was considering implementing this feature myself until I saw someone beat me to it! Thank you :]

[junlarsen]

Thank you, when do you think you'll be able to make a release for it?

[juhaku]

Thank you, when do you think you'll be able to make a release for it?

I could make a release soon. There are also some other changes that has been quite a long time there waiting to be released.

[junlarsen]

Thanks for the release!