introduce mode where asynchrony is determined solely from method's return type

[Ladicek]

Currently, we determine whether a method is asynchronous (for the purpose of fault tolerance) based on an annotation (MP FT @Asynchronous or SRye Common @Blocking/@NonBlocking), together with the method's return type. This is to maintain close correspondence to MP FT.

However, there's a lot of libraries and APIs, especially in Quarkus, that determine asynchrony solely from method's return type: MP RestClient, MP Reactive Messaging, JAX-RS, etc. Fault Tolerance is an outlier.

We could introduce a mode (based on a global config property) that doesn't require the annotation and just looks at the return type. That would be off by default (there's a test in MP FT TCK that verifies that a CompletionStage-returning method without @Asynchronous is handled synchronously), but could be turned on by a user.

Quarkus could potentially turn it on by default, and users could turn it off. That would of course technically be a breaking change, but probably for the better.