Enhancement: Add Java code sample for listing Gmail messages

[emrila]

Description

On the List Gmail messages guide, there's a code snippet provided in Python, but currently no corresponding Java example.

While not a bug, the goal is to improve coverage and consistency in the programming languages presented as snippet examples across the Gmail API documentation. Adding a Java example to match the existing Python list_messages sample can make it faster for Java developers to grasp the logic and get the gist of how it works. This simply smooths out the process so they can effectively move attention to what the next move should be when using the API in their own solutions.

Proposed Solution

Add a ListMessages Java sample for the List Gmail messages guide (and corresponding unit tests) to the repository.

Related PR

See PR #1748.