sea-orm-cli: allow skipping impl ActiveModelBehavior for generated entities

[waynr]

This PR adds an option that enables users to skip generating the

impl ActiveModelBehavior for ActiveModel {}

by passing the --gen-impl-active-model-behavior false flag to sea-orm-cli generate entity.

This is intended to allow for an easy and idiomatic-for-rust alternative to #1931 which I described in more detail in a related discussion post

I don't necessarily think this should close out #1931, but I do think that the complexity of the solution described in that issue merits some consideration given the available alternative.

[waynr]

The current state of this PR only adds the CLI flag that maintains the current default behavior and updates EntityWriter test cases to include the new option. If maintainers are open to this approach I'd be happy to add some new test cases to validate the false code path.

[dave42w]

I think this is an excellent solution.

[waynr]

Something I didn't realize when I opened this PR is that it may be a little problematic since it's all-or-nothing whereas it may be desirable for the empty impl blocks to exist for some of the generated entities since some ActiveModel trait methods have ActiveModelBehavior as a trait bound: https://docs.rs/sea-orm/latest/sea_orm/entity/trait.ActiveModelTrait.html

[tyt2y3]

The implementation looks good, and this feature is definitely useful!

[jondot]

Hi this great, much needed, can we merge this?

[tyt2y3]

@waynr do you have anything to amend? I can probably merge it as is