Should be possible to deserialize unknown enum values

Consider the following Proto file:

message Foo {
  MyEnum my_enum = 1;
}

enum MyEnum {
  MY_ENUM_UNSPECIFIED = 0;
  MY_ENUM_BAR = 1;
  MY_ENUM_QUX = 2;
}

I'm interested in receiving JSON-encoded Foo messages where the enum might have more values added to it later, without me having to update my Proto file and re-generate code. Both of these JSON blobs deserialize fine: {"myEnum": "MY_ENUM_UNSPECIFIED"} and {"myEnum": 1}. However, these two won't work: {"myEnum": "MY_ENUM_BUX"} and {"myEnum": 100}.

It would be great if some sort of mechanism could be added to tolerate unknown enum values.

Start by locating the generated serde handling for prost enum values and compare its behavior for known numeric and string inputs. Determine how unknown enum names and numbers should be represented without updating the .proto, then add coverage for both cases; done means those JSON forms deserialize successfully while existing behavior remains intact.