Make SearchQuery open for extension

Currently, it is not possible to extend SearchQuery in a reasonable way, since it's fields and inner classes are all private and the execute methods are final.

But there are be scenarios where extending SearchQuery would be quite useful, for example

• if the developer needs the full control over the generated JPQL query
• if it is necessary to do some kind of post-processing/mapping of the results
• if custom finder methods should be provided

Therefore, the fields and inner classes should use protected modifier, and the execute method shouldn't be final.