Allow `#[builder(on(Vec, default))]`

[Kriskras99]

I want to add builders to all types generated by prost-build.

All fields are optional, but Prost will generate a Vec instead of an Option<Vec>.
So it would be useful to do #[builder(on(Vec, default))], so those fields are optional too.

[Veetaha]

Makes sense, we should definitely have smth like this. The type pattern for Vec is Vec<_>, so it should be #[builder(on(Vec<_>, default))]. The default attribute in this position can only be a boolean switch and nothing more until #152