Unify all Graph Dataset types

[Dsantra92]

Related to: #169

Added the GraphDataset struct which can replace all custom structs for each Data source.

The metadata will have fields like OGBDatasets metadata, including name, source, etc. We also can add data citations. This is also required for saving datasets in parquet format, as defining storing method for each struct is troublesome.

@CarloLucibello if you can review the initial struct, I can start with replacing the data source structs.

[codecov-commenter]

Codecov Report

Merging #173 (f053ef4) into master (e8ec56e) will decrease coverage by 0.13%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
- Coverage   32.03%   31.90%   -0.14%     
==========================================
  Files          42       42              
  Lines        2132     2141       +9     
==========================================
  Hits          683      683              
- Misses       1449     1458       +9     

Impacted Files	Coverage Δ
src/graph.jl	44.30% <0.00%> (-5.70%)	⬇️

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[CarloLucibello]

I'm not okay with replacing every graph dataset with a unique structure. At least we should first have a long discussion about that.

Each dataset in MLDatasets used to be its own module, then with a large and breaking change datasets became types. I don't see enough compelling reasons to go through another heavily breaking change.

Having datasets as their own types means we can customize their behavior with specialized methods, we don't want to lose that flexibility.

We can still expose a GraphDataset for the user's convenience in defining their own datasets, similarly to the containers in https://github.com/JuliaML/MLDatasets.jl/tree/master/src/containers

[Dsantra92]

I didn't look into containers enough, I will look into that.