Support generating builders from traits and trait impls

[Veetaha]

We need some syntax to allow developers to generate builders from traits and trait impl blocks. This way it should be possible to define a trait that uses builder syntax for its methods.

The main problem here is that trait declarations and trait impl blocks are syntactically separated. We can also have multiple impl blocks. It would be awesome if all the impl blocks didn't have to redeclare the trait methods' parameter lists. The macro should probably generate a struct with input parameters that is then passed into the trait methods.

This all has a bunch of nuances (e.g. support for dyn Trait objects, methods without self). This feature requires a huge design effort.

A note for the community from the maintainers

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[dzmitry-lahoda]

assuming it is possible to do, when it is useful?

[Veetaha]

As an example, the current Hashicorp Vault API uses functions with lots of positional parameters and it always accepts an impl Client.

If only we had a way to generate builder syntax for that Client trait, that would make it possible for vaultrs to use the syntax like this:

let client: impl Client = /**/

client
    .create_certificate()
    .mount("...")
    .cert_name("...")
    .aws_public_cert("...")
    .send()
    .await?;