chore(spanner): add example for Mutable Credentials

[ldetmer]

This provides an example of how to use mutable credentials.

[gemini-code-assist]

Code Review

This pull request introduces a new Java snippet, MutableCredentialsExample.java, which demonstrates how to implement dynamic credential rotation in Google Cloud Spanner using MutableCredentials and a scheduled executor to monitor file updates. The review feedback suggests improving the example's usability by passing the credentials path as a method parameter and enhancing code readability by using an import for AtomicReference instead of its fully qualified name.

[snippet-bot]

Here is the summary of changes.

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• java-spanner/samples/snippets/src/main/java/com/example/spanner/MutableCredentialsExample.java:19, tag spanner_mutable_credentials

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