[2.0.0 REVIEW] Thoughts on a selection expression for models

In regards to the [oneOf](https://www.asyncapi.com/docs/specifications/2.0.0-rc1/#a-name-operationobject-a-operation-object) support for a `message` within the `Operation` object:

Having the concept of "multiple message shapes can be received over this channel operation" is very useful - Perfect!

However, what indicates to the recipient which message should be chosen at runtime?

Within AWS API Gateway, we support a concept of a `modelSelectionExpression` on our Routes - I think this roughly aligns with your `Runtime Expression` concept at a high level. Do you think this sort of runtime expression should be added within the `Operation`?

-Jason
AWS API Gateway

Review the AsyncAPI 2.0.0 Operation and message/oneOf definitions, then compare them with AWS API Gateway's modelSelectionExpression and the Runtime Expression concept. A complete contribution would need a settled proposal for where selection belongs and updated specification text and examples, but this issue does not name files or tests.

Written by the indexing model from the issue text.