Server: add wrapper to fetch data from the graph

[0xGabi]

Reuse the scheme we were using on Connect to have a basic wrapper that only supports query data from the graph.

[nivida]

Added some comments. I think many of them are just popped up cause I'm not sure what the standards are here at Aragon.

Overall: I would probably combine those two TheGraph classes to a TheGraphServiceClient. 🤷‍♂️

[nivida]

Not sure how the structure is but we will use I think anyways all of those functions here. This means it will get bundled anyways and we could forward the creation of nice TS classes.

What do you think about creating out of it a value object class?

const network = new Network('mainnet');
console.log(network.chainId);

const network = new Network(netObj);
console.log(network.chainId);

console.log(network.toObject());
> { ... }

[codecov-io]

Codecov Report

Merging #59 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #59   +/-   ##
=======================================
  Coverage   36.76%   36.76%           
=======================================
  Files           8        8           
  Lines         204      204           
  Branches       30       30           
=======================================
  Hits           75       75           
  Misses        129      129           

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