PSA: the generated `clone()` and `copyWith()` methods are deprecated

[devoncarew]

As a general PSA, the generated MyMessage.clone() and MyMessage.copyWith(Function updates) methods are deprecated. These methods can add significant overhead to your binary; we plan to remove in a future major version release of this package.

Instead of MyMessage.clone(), call GeneratedMessageGenericExtensions.deepCopy().

Instead of MyMessage.copyWith(), call GeneratedMessageGenericExtensions.rebuild().

We'll link to this issue from the generated source.