shim: make sure it's connected to sugondat-node

[pepyakin]

Shim expects a node URL to connect to. It's common to run several nodes on one machine and I think it's not impossible to connect to a wrong node, e.g. relay chain node.

Using subxt we can validate that the blobs pallet is present upon establishing connection.

[rphmeier]

We can do this in sugondat_rpc::Client::new, and make that function fallible: "connected to a Substrate node that is not Sugondat"