[sdk-platform-java] Update the generated samples to reflect client lifecycle best practices

The generated samples in each GAPIC client library show use cases of try-with resources. Example: https://github.com/googleapis/google-cloud-java/blob/7346a852311cdcb958ef57066eaab8679f02a64c/java-admanager/ad-manager/src/main/java/com/google/ads/admanager/v1/NetworkServiceClient.java#L325

While this is a good example as a sample to show how a client is created and closed without incurring memory leaks, it may provide our users with an incorrect idea of how to create clients. We expect client libraries to be long-lived and not to be re-created/ closed repeatedly.

We should update the samples with a few options:
1. Samples are updated to not use try-with-resources
2. Samples have wording to describe that they are for example purposed and link to the official docs
3. ... Other options?

High Level Changes:
The samples from the GAPIC clients are generated from the generator. Based on the option we choose, we'll have to update the generator accordingly.