support boxed(type_name)

[xxchan]

Currently we only support boxed(field_name). Sometimes we may just want to box a type in all occurances.

example:

oneof typ {
  A a = 1;
  B b = 2;
  // ...
}

message A {
  LargeMessage m = 1;
  // ...
}

// LargeMessage may even occur multiple times.
message B {
  LargeMessage m1 = 1;
  LargeMessage m2 = 2;
  // ...
}

This can easily lead to large_enum_variant problem https://rust-lang.github.io/rust-clippy/master/index.html#large_enum_variant (Our real example is here risingwavelabs/risingwave#19906)

One solution is just boxing all enum variants #1209

Another solution is to box LargeMessage types in all places instead, which will need a new feature by prost-build. It will be troublesome to specify all boxed(field_name)