feat: add iam authn example for mysql

[shubha-rajan]

• I have followed Sample Format Guide
• pom.xml parent set to latest shared-configuration
• Appropriate changes to README are included in PR
• API's need to be enabled to test (tell us)
• Environment Variables need to be set (ask us to set them)
• Tests pass: mvn clean verify required
• Lint passes: mvn -P lint checkstyle:check required
• Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only
• Please merge this PR for me once it is approved.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• cloud-sql/mysql/servlet/src/main/java/com/example/cloudsql/ConnectorIamAuthnConnectionPoolFactory.java:19, tag cloud_sql_mysql_servlet_auto_iam_authn

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