Add support for deploying and testing against locally running node

[zees-dev]

Description

It would be very helpful for the sway-playground to support local testing; i.e. pointing to a local node for deployment and calling/querying contracts.
This is generally basic functionality which exists on playgrounds for other ecosystems (solana, ethereum etc.).

[zees-dev]

In fact, we should just allow pointing to any custom URL (where it defaults to a local node's URL).
We would need to also add some sort of indicator displaying the that node is online/connectable*.
The connect button should be disabled if the node URL cannot be reached.