Allow for QueryBuilder to be easily replacable

[karelklima]

While it is possible to replace QueryBuilder component in LDkit during runtime, it is not straightforward and the API for this is not documented. Users should be able to define a custom QueryBuilder so that they can modify SPARQL queries or perhaps implement custom query languages more easily if needed.

[karelklima]

The original idea for replacable QueryBuilder was to potentially support GraphQL, but that is very far from the original intention of LDkit and the use case does not justify the added complexity of LDkit.