Support custom sessions in GQL and REST APIs

[lukashornych]

Currently, GQL and REST APIs create new sessions for each request. This is usually fine but e.g. in evitaLab (lukashornych/evitalab#160), we will need to manage single session for WARMING_UP catalogs.

Therefore it would be helpful if GQL and REST APIs could support session ID on the input to override the default logic of creating adhoc sessions.