Add a utility method to turn a legacy throwable-based retryWhen function to a Retry

[simonbasle]

Motivation

Since 3.3.x, retryWhen(Function<Flux<Throwable>, Publisher>> is deprecated, and it is removed in 3.4.0-M2.
However, some users do still rely on reactor-extra 's reactor.retry.Retry builder, which implements Function rather than Retry.
The Retry.from(Function<Flux<RetrySignal>, Publisher>) utility method only helps halfway for a migration, because it is RetrySignal based and not Throwable based.

Desired solution

Adding a second utility adapter method, Retry.fromThrowable(Function<Flux<Throwable>, Publisher>), that performs the full adaptation.

Considered alternatives

A current way to adapt the code is to map the RetrySignal to its failure:

Function<Flux<Throwable>, Flux<Long>> retryLegacyFunction;
Retry adapted = Retry.from(rs -> rs
    .map(RetrySignal::failure)
    .transform(retryLegacyFunction));

Additional context

See reactor/reactor-addons#231 for an example of an issue where users have trouble finding the correct migration path.