Provide definition for the reply

In current Spec v3 pre-release the `reply` appear first time as `operationObject` field:
https://github.com/asyncapi/spec/blob/abfae495b0fbb88fccd56f8099e466a83f4b76ab/spec/asyncapi.md?plain=1#L843
with the description `The definition of the reply in a request-reply operation`.
But Spec v3 has no definition or describing what is meant under `request-reply operation`.
@jonaslagoni wrote the [blog post](https://eventstack.tech/posts/asyncapi-v3-request-reply) describing what it could be, but the blog post is not the Specification. Blog post could be a spice for a cake, but not the cake.

I suggest to provide definition of `reply` in the [Definitions](https://github.com/asyncapi/spec/blob/next-major-spec/spec/asyncapi.md#definitions) section, maybe as subpart of Message definition.

Start in spec/asyncapi.md at the Definitions section and the operationObject field where reply first appears. Read the linked request-reply blog post for context, then define reply and request-reply operation in the Specification so the terminology and intended meaning are explicit.

Written by the indexing model from the issue text.