Create a JsonbConfig from an existing Jsonb instance

It is occasionally necessary to clone an existing `Jsonb` instance, and optionally override a specific setting. Would it be possible to add a method like `Jsonb#getConfig` that returns `JsonbConfig`?

**Usage example**

```java
JsonbConfig customConfig = originalJsonb.getConfig()
.withPropertyNamingStrategy(new CustomNamingStrategy());
Jsonb customJsonb = JsonbBuilder.create(customConfig);
```

(I replaced the example. The original was drawing attention away from the central point.)

Start by reviewing the Jsonb and JsonbConfig API definitions and the existing comment discussion. Define the behavior for creating a JsonbConfig from an existing Jsonb and overriding a setting; done means the proposed API preserves the original configuration while allowing the usage shown in the issue.

Written by the indexing model from the issue text.