Add source-specific interface

[remilvus]

Description

We need interfaces that allow downloading files in GTFS format in a predictable manner.
Each interface should work with one source. The interface should return files in a container that is independent of the source.

What needs to be done:

• [x] Data containers for file contents.
• [x] Interface for London.
• [x] Interface for Krakow (buses).
• [x] Interface for Krakow (trams) .

Extra remarks:

• Krakow's feed consists of two feeds - one for trams and one for busses. The former is suffixed with "_A" and the latter with "_T".
• [x] Files are not downloaded if their ETags are not changed.
• [x] GTFS real-time files are downloaded as Protocol Buffers, and not in the json format.