Gracefully handle runtime upgrades while connected to a node

[jsdw]

During a runtime upgrade, things like the spec/runtime version will potentially change. The subxt Client downloads these when it first connects to a node, and after an update they might no longer be valid. This would lead to things like transactions (which rely on these numbers lining up between subxt and node) no longer working. The current workaround would be to create a new Client.

Can/should we subscribe to something like state_subscribeRuntimeVersion when the Client connects to a node, to make sure that any changes to these values (we care about transaction_version and spec_version) are kept in sync?

The metadata might also change; could also spot this and update our stored metadata? With things like the static validation work, this would make sure that a long running client can properly warn about differences (as well as make sure that constants and such as obtained from the metadata are still correct).