Make request body required by default

As per @EricWittmann’s request following a discussion in https://github.com/smallrye/smallrye-open-api/issues/110#issuecomment-493473681 I propose to make the required property of @RequestBody true by default

because more often than not you expect the request body and depend on it

For reference, this is the current state: https://github.com/eclipse/microprofile-open-api/blob/49e31ea4bf6da48f8491327e1df7f4e47febfc3a/api/src/main/java/org/eclipse/microprofile/openapi/annotations/parameters/RequestBody.java#L59

[EricWittmann]

After further discussion in today’s bi-weekly specification meeting, we have reached a consensus that we will, in fact, change the default value of the reqired property to true. This does make sense in the framework of default values of annotations, but it represents an exception relative to other annotation property default values. An explanation of what I mean by that can be found here:

https://github.com/eclipse/microprofile-open-api/issues/356#issuecomment-502763082

The tldr is that we’ll make the following changes:

• Change the default value of this to true
• Update the javadoc to explain why it’s true when most other boolean property defaults are false
• Update the text of the specification to better explain default annotation properties more broadly
• Maintain a table of exceptions (like this one) where appropriate
• Add a TCK test to ensure that implementations properly handle this case

[MikeEdgar]

Placing this in the 4.0 milestone as it requires implementations to generate required: true for request bodies which is a change in behaviour.