Add export/import util - #100

I have a few comments/concerns:

1. Theoretically speaking, I supposed it is enough to count out_edges of every vertex to get all edges? When I thought about it, I concluded that if a vertex has in_edge, then that edge is also an out_edge of another vertex.
2. I noticed an issue when exporting to a local JSON file. I need to change permissions locally in order for this to work. Not sure if that could be handled better.
3. I still didn't write any tests since I'm not sure how to write them since the return type is None (an empty mgp.Record).
4. I know putting everything in the dictionary might seem silly, but mgp.Vertex.properties is not JSON serializable, as well as some other types, so I had to go this way until there is some kind of toJSON() method in mgp.

1. I think this is absolutely alright thing to do
2. Permissions and how do they work with memgraph user is something that @antonio2368 is more familiar with. It definitely has to do something with it
3. You won't be able to write e2e tests, but a working unit test (checking only the functionality of methods in a module) would work just fine. But it would require moving the functions to the python/mage directory
4. This would be an interesting suggestion for mgp module. I don't think it should contain JSON, but a transformation to Python Dict would be helpful

1. Yaay!
2. Okay, I will ask Antonio :)
3. I'll try writing the unit test and will let you know when it's done
4. Will make an issue on memgraph repository and see how it goes

Asked Antonio about permissions, and the possible solution is:
Create folder "folder_name" which will be the folder for exported files. With
sudo chown memgraph "folder_name"
give permission to the user memgraph, since that user has started the process.
After that, memgraph will be able to write to the files within that folder.
I tried that and it works, it just has to be described correctly in the documentation.

I added test here, and I don't see that I should do any error handling here, since I am only communicating with memgraph and writing to file. Everything necessary for the output file is defined in the docs, mentioned above.

Thank you @g-despot, totally forgot about that!