jersey-apache-connector: Add support for custom ServiceUnavailableRetryStrategy

I was previously using Jersey 1.x, where I could manually configure the Apache HttpClient. Part of this configuration was a custom ServiceUnavailableRetryStrategy, which is required by my code to handle rate limiting on a remote system, and handle authentication.

I have now upgraded to the latest Jersey version, but unfortunately the new jersey-apache-connector doesn't allow me to configure a custom ServiceUnavailableRetryStrategy. For now I have simply copied the relevant connector code in my project, and added a property for configuring the ServiceUnavailableRetryStrategy, similar to the recently added property for RetryHandler.

Can you please add support for this to jersey-apache-connector, such that I no longer need a customized version?

Simple approach would be:

* Add the following to ApacheClientProperties:
```java
public static final String SERVICE_UNAVAILABLE_RETRY_STRATEGY = "jersey.config.apache.client.serviceUnavailableRetryStrategy";
```
* Add the following to ApacheConnector:
```java
final Object serviceUnavailableRetryStrategy = config.getProperties().get(ApacheClientProperties.SERVICE_UNAVAILABLE_RETRY_STRATEGY);
if (serviceUnavailableRetryStrategy != null && (serviceUnavailableRetryStrategy instanceof ServiceUnavailableRetryStrategy)) {
clientBuilder.setServiceUnavailableRetryStrategy((ServiceUnavailableRetryStrategy) serviceUnavailableRetryStrategy);
}
```

As a more structural solution, it would be nice if callers can have direct access to HttpClientBuilder and other HttpClient-related functionality to allow for full customization of the connection, to avoid having to add properties for every possible HttpClientBuilder configuration.