Custom retry handlers

[cgillum]

This PR completes the retry policy feature for Java by allowing developers to implement custom retry policies using code. It's intended for use-cases where declarative retry policies aren't flexible enough.

Example syntax

TaskOptions optionsWithRetryHandler = TaskOptions.FromRetryHandler(retryCtx -> {
    // Retries MyException or any sub-class of MyException
    if (retryCtx.getLastFailure().isCausedBy(MyException.class)) {
        return true;
    } else {
        // For any other exception type, retry up to 4 times
        return retryCtx.getLastAttemptNumber() < 5;
    }    
});

TaskOrchestration  = ctx -> {
    ctx.callActivity("MyFlakeyActivity1", null, optionsWithRetryHandler).get();
    ctx.callActivity("MyFlakeyActivity2", null, optionsWithRetryHandler).get();
};

Code changes:

• New public classes to support custom retry logic
• Refactored integration tests to share common logic
• Added support for non-retriable errors
• Fixed bug with backoff coefficient

Resolves https://github.com/microsoft/durabletask-java/issues/18