Add possibility to filter by one field with `QueryRequest`

[dmytro-kashcheiev]

In the current implementation to query something with a single field filter, CompositeFilter.all([...]) should be used:

var queryResult = client
    .select<Entity>()
    .where(all([
  	eq("field", "value"),
    ]))
    .post();

Consider adding QueryRequest<M> where(Filter filter) method to QueryRequest.