Form system improvements

[m4tx]

Problems

There are some problems with the current Form system:

1. It's not possible to specify the widgets to be used by a specific field (for example, String might be constructed using a single-line text field, or an HTML text area)
2. There is no difference between a generic form and an admin form. The admin panel might want to define its own form fields - for instance, to allow modifying foreign key target records easily. Users might also want to implement their "form contexts" to have different settings for different kinds of forms.

Because of the above, the form system needs to be improved.

Solutions

Problem 1

The simplest idea would be to allow specifying the widget using #[form] attributes:

#[form(opts(max_length = 100))]
title: String,
#[form(widget = cot::form::widgets::TextArea)]
description: String,

This is similar to what Django implements.

Problem 2

An idea is to introduce "form types", which would be a piece of metadata encoded in the type system, specifying what kind of form are we dealing with.

Instead of having a single #[derive(Form)] macro, we could have an attribute macro #[form] instead, allowing us to specify the type of the form, such as #[form(type = AdminForm)], for instance. Then, the Form trait would instead become Form<FormType> trait instead, allowing multiple Form implementations for the same struct. Then, each type could define the form type it has a form widget for, allowing us to definite completely different widgets for generic and admin forms, and for the users to specify their own form types as well.

[ElijahAhianyo]

Yeah, I like the #[form] attribute macro solution