Make FormFrom a separate crate.

[Tomperez98]

Existing Functionality

Currently, the trait FromForm is part of the API framework. But the data validation functionality comes really handy for other types of applications such as (CLI) applications.

Suggested Changes

Move the FromForm trait a a separate crate so the logic can be used for different types of applications, other than APIs.

This would be similar to the core functionality Pydantic provides. Pydantic + FastAPI is a great combination, but the validation logic Pydantic provides can be used even if FastAPI is not installed

[Tomperez98]

Maybe this covers it.

https://github.com/Keats/validator

[SergioBenitez]

The FromForm trait is very specific to Rocket. The validation parts are somewhat specific to Rocket but could be extracted out. I think that would be great, but doing so isn't really within the scope of the Rocket project itself, so I'm closing this issue. Feel free (of course!) to take any of the code and transform it into an external crate!

Also see #1915.