feat: add context manager capability to subscriber - #32

Fixes #4.

This PR gives users a more handy way of closing the subscriber's underlying channel.

How to test

• Make sure that a topic has a few messages published to it to avoid the issue with a dubious DeadlineExceeded response.
• List the currently open sockets by Python:

  $ lsof -i -n | grep python
  

• Run the code snippet from the ticket description in debugger, but by using the subscriber client as a context manager:

  with subscriber:
      ...

  Make sure to place a breakpoint after the with block.
• After the messages have been pulled and the context manager exited, list the open sockets again.

Actual result (before the fix):
Subscriber client cannot be used as a context manager. Using it without the context results in open sockets being leaked until the Python process terminates,

Expected result (after the fix):
Subscriber client releases the open sockes after exiting the context management block.

Things to consider/discuss

• I experimented by adding a _close flag to the client and additional logic that would raise an error if any of its methods are used after the client has been closed. Since a lot of the methods are copied from the generated SubscriberClient GAPIC class, that extra logic would bloat the code without that much of a gain (the GRPC channel itself already raises an informative "channel closed" error).
• I did not implement the __del__() method that would call close() automatically, as this could bring its own set of potential problems.
• While already at it, should we add similar logic to the publisher client, too? The latter, too, opens two sockets through its underlying GRPC channel.
• Update code samples in docs to demonstrate the context manager capability as a recommended practice?

PR checklist:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)