Calls and subscriptions should accept a context

[philipstanislaus]

Calls should accept a context so they can be cancelled and can receive a timeout.

Subscriptions need a bit more investigation – a context could be used to cancel or timeout the subscription, but the current subscription logic already exposes an Unsubscribe method that can be used for both purposes.

[johnuopini]

I definitely need this, i create a client every 2/3 minutes to get data from chain, if the client gets stuck i have no way to cancel that call and retry, i haven't found any fix so far, having an additional constructor that accepts a context or add a context to the call would be great

[cdamian]

The Client interface will be updated here - #366 to include the CallContext method. We will not update any RPC clients as of now.