feat: add induced_subgraph functionality

[askorupka]

This PR introduces induced_subgraph functionality to match Graphs interface.

As per @CarloLucibello comment in the #495

Notice that we already have the very similar function sample_neighbors in GNNGraphs.jl. Let's implement induced_interface as well in order to be Graphs.jl compliant.

[askorupka]

I'm aware that we shouldn't assume ndata are called x and also extract edata & I've marked it as ##TODO
But I've got a question related to this discussion #497 (comment)
@CarloLucibello what is the best way to implement it as getobs doesn't work

julia> getobs(g.ndata, 1)
ERROR: UndefVarError: `getobs` not defined
Stacktrace:
 [1] top-level scope
   @ REPL[8]:1