Improvement: Make interfaces serializable

[thisdotnull]

Resilience4j version: 1.7.1

Java version: 1.11

What am I trying to do?
We are using Apache Flink which requires all classes (user-defined functions) and their members to be serializable. I'm trying to decorate all the functions with a Retry.of(Function<I, O>) and subscribe to its events.

The Problem
The Function decorated with Retry.decorateFunction(Retry, Function) is a class-level field to avoid duplicate subscriptions to events.
The flink program fails to bootstrap because the class-level field Function is not serializable. This is because the Retry.decorateFunction(Retry, Function) captures the Retry param(see this).

While the configuration classes are marked serializable (thanks to #1046), the corresponding interfaces like Retry and their implementations like RetryImpl are still not serializable.

Current Workaround
To overcome this, I have to mark the decorated function as transient and specify deserialization behavior by overriding the readObject(ObjectInputStream).

class CustomWrapper<I, O> {
    private Function<I, O> actualFunction;
    private transient Function<I, O> decoratedFunction;

    public CustomFunction (Function<I, O> someFunction) {
        this.actualFunction = someFunction;

        // instantiate the Retry
        RetryRegistry registry = RetryRegistry.ofDefaults();
        Retry retry = registry.retry("name2");
        subscribeToRetryEvents(retry);

        // The returned function captures `Retry` passed in params.
        this.decoratedFunction = Retry.decorateFunction(retry, someFunction);
    }

    // Override default deserialization behavior
    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
        stream.defaultReadObject();

        // Re-wire the transient field decoratedFunction
        RetryRegistry registry = RetryRegistry.ofDefaults();
        Retry retry = registry.retry("name2");
        subscribeToRetryEvents(retry);
        this.decoratedFunction = Retry.decorateFunction(retry, this.actualFunction);
    }

    public O doSomething(I input) {
        return decoratedFunction.apply(input);
    }
}

Proposed Solution
Mark the interfaces as serializable so that all the implementations are serializable by default.