Support shared #[builder] configuration in #[bon] macro at the impl block level

[Veetaha]

In some cases, developers would like to share the #[builder(...)] configuration across all their methods in the impl block, or even across all impl blocks.

Today, the users of bon need to copy-paste the same builder configs across all methods inside of the impl block manually, which isn't convenient when the number of methods is big and growing.

The proposed syntax is the following:

#[bon(builder(on(...), finish_fn = ..., derive(...))]
impl Foo {
    #[builder] // will inherit configs from `#[bon(builder(...))]`
    fn method(/**/) { /**/ }
    
    // Won't have a `#[builder]` generated
    fn regular_method(/**/) { /**/ }
}

This way the developers can move their configs to the top of the impl block to prevent code duplication at the entire impl block level. If the developers want to share these configs between multiple impl blocks they can easily create an attribute alias with macro_rules_attribute::attribute_alias.

Once we have this feature, the documentation for the "Shared Configuration" pattern needs to be updated to propose a solution for sharing configs for associated methods using the new syntax.

A note for the community from the maintainers

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