Make an unifying type for all graph datasets

[Dsantra92]

It would be good to have something like:

struct GraphDataset <: AbstractDataset
    name::String
    metadata::Dict{String, Any}
    graphs::Vector{Graph}
    graph_data::Union{Nothing,NamedTuple}
end

Instead of individual struct for each dataset. Unifying them would reduce a lot of code duplication.

Also, metadata fields can be adopted from the OGB convention.

cc: @CarloLucibello

[Dsantra92]

Closing this as not planned.