Add Close() error to bindings

[pkedy]

Currently, the PubSub interface contains a Closer() error function so that the component can cleanly shutdown when daprd is stopped. This does not exist in bindings and that has resulted in binding implementations are having to use signal.Notify in order to stop a subscription, etc.

Instead, PubSub and Binding interfaces could embed io.Closer. Many of the pubsub components have moved to using an internal content.Context for backoff/timeout purposes. The Close method should call the cancel func returned by context.WithCancel in order to idiomatically shutdown goroutines, etc.

This likely involves a change to dapr/dapr (most likely in runtime.go) to invoke Close on shutdown. It's also possible for components to optionally implement io.Closer so long as daprd checks for that and invokes accordingly. Our documentation on how to build a component would just need to make that clear.

[pkedy]

This was resolved in dapr/dapr. Opening new issues for each component as I run into "interrupt signal" based cleanup logic.