fix(pubsublite): wire user context to api clients

[tmdiep]

Passes the context provided for New(Publisher|Subscriber)Client through to gapic clients. This will allow users to pass through a cancelable context, which they can use to quickly terminate the publisher and subscriber clients. This will give users more options to handle various scenarios (e.g. force immediate shutdown, hanging clients).

It was originally not passed through to ensure that we can perform graceful shutdown (e.g. flush pending published. messages, commit cursors, etc), but there are situations where attempts to connect to the server can have long delays.

All other libraries just pass the context provided to the NewClient constructors through to the gapic clients, so we'll be consistent.