Hold chain api connection at the top level in the IndexerClient in `@sdk`

[royvardhan]

Ref: #29 (comment)

We are currently using getChain function inside client.ts whenever we need hyperbridge methods, this triggers substrateChain.connect(). We could hold the connections in the top level in the client and use it instead, maintaining proper initialisation of the connection and cleanup.

We can also then use these connection instances in our tests.