feat: All changes required to change library to use ApiException and gapic internals

[dpcollins-google]

This converts the internals of the client library to use GAPIC generated clients, as well as the api surface to raise ApiException as opposed to grpc StatusException.

fixes #180

fixes #148

This does not throw RESOURCE_EXHAUSTED at high partition counts with the following code when removing RESOURCE_EXHAUSTED from the list of retryable errors:

public class PublisherTest {
  public static void main(String[] args) throws Exception {
    Publisher publisher = Publisher.create(PublisherSettings.newBuilder()
        .setTopicPath(TopicPath.parse(<400 partition topic>)).build());
    publisher.startAsync().awaitRunning();
    publisher.publish(PubsubMessage.newBuilder().setData(ByteString.copyFromUtf8("abc")).build()).get();
    publisher.stopAsync().awaitTerminated();
  }
}

[anguillanneuf]

Thanks for making this PR. I didn't review any of the tests. Here are some issues I found:

1. This comment about setting stub on the publisher needs an update:
   

   java-pubsublite/google-cloud-pubsublite/src/main/java/com/google/cloud/pubsublite/internal/wire/PublisherBuilder.java

   Line 56 in 7b6bd05

   * <p>Custom batching settings and a custom GRPC stub can also be set.

2. I'm not sure if you intended google-cloud-pubsublite/src/main/java/com/google/cloud/pubsublite/internal/testing/UnitTestExamples.java to be there.
3. Sporadic class descriptions in google-cloud-pubsublite/src/main/java/com/google/cloud/pubsublite/internal/wire/.
4. Changes in pubsublite-beam-io/ and pubsublite-kafka-shim/ should be their own PRs.
5. Changes to grpc-google-cloud-pubsublite-v1/ should come from synthtool pulling from the source.
6. Update samples in a later PR.
7. Also please resolve the conflicts with main.

I haven't tried the code to see how they impact samples myself. I can do that next week.

[dpcollins-google]

Thanks for making this PR. I didn't review any of the tests. Here are some issues I found:

Replies to each point below.

1. Done.
2. Yes, since this needs to be accessed from other modules.
3. This is not a public API, thus it can be put off until later as the only users of these classes would be maintainers.
4. No, since they would not compile otherwise.
5. I've reverted these. They were unintentional.
6. No, we've discussed this previously. For the integration tests to pass as currently set up, samples changes need to be made in the same PR, since the blocking presubmit is the snapshot one.
7. Done.

[anguillanneuf]

3. It's in the public documentation, either we add more documentation there, or we choose to hide it. Can be in a later PR.
6. Ok. Make sure to make a new release soon after this PR is merged then.