Fix rxjava context propagation for ObservableFromCallable

[jaydeluca]

Fixes #14382

When using Observable.fromCallable() with different schedulers (.subscribeOn()/.observeOn()), the context was not being propagated to the callable execution.

ObservableFromCallable is an internal class with a private callable, so I needed to do some reflection in the wrapper to make it work, but we now intercept and wrap the callable before it gets scheduled.

[trask]

@jaydeluca can you check if the test latest deps failure is related?

HibernateReactiveTest > initializationError FAILED
    java.lang.NoSuchMethodError: 'io.vertx.core.Future io.vertx.core.Context.executeBlocking(io.vertx.core.Handler)'
        at io.opentelemetry.javaagent.instrumentation.hibernate.reactive.v2_0.HibernateReactiveTest.setUp(HibernateReactiveTest.java:78)

> Task :instrumentation:hibernate:hibernate-reactive-1.0:javaagent:hibernateReactive2Test FAILED

[jaydeluca]

@jaydeluca can you check if the test latest deps failure is related?

@trask that testlatestDeps is happening on all PRs and I'm able to reproduce on main locally. There was a hibernate-reactive release that pulled in a newer version of vertx and changed some methods we use in our tests, so it should be unrelated to this PR