Return specialized Vespa connection depending on the ApplicationPackage inheritance

[thigm85]

When deploying a specialized application package such as:

from vespa.gallery import TextSearch 

app_package = TextSearch(id_field="id", text_fields=["title", "body"])

the instance returned by the deployment method could be specific to this specialized application, such as VespaTextSearch, inheriting from the base class Vespa.

Different app use cases have different needs and pyvespa currently lacks a pattern to encode those needs. Just to give another example, this would be useful to natively support TextImageSearch and similar use cases.

[thigm85]

This is no longer in the pyvespa scope.