feat(sdk/go): Include a shutdown signaller for closing a client

[gregfurman]

Description

Fixes #3283

This PR introduces a shutdown utlity that will allow a Client to close all pending requests via context cancellation.

Since there is no way to cancel a hanging NewWorkflow request without introducing breaking changes, this approach allows us to forcefully stop all requests with client.Close()

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

What's Changed

• Added a new shutdown.Signaller utility in internal/shutdown/shutdown.go that allows us to derive multiple contexts and cancel them when a shutdown call is made.
• Included a Close() error method on the Client interface, which should ungracefully cancel all currently running requests.

[vercel]

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