Add getCallerIdentity as an STS simple method

[kiiadi]

Description

Mark GetCallerIdentity as a verified simple method.

Motivation and Context

GetCallerIdentity does not require any input parameters and should be exposed as a simple method on the StsClient

Testing

mvn install -pl :sts

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed
• A short description of the change has been added to the CHANGELOG

License

• I confirm that this pull request can be released under the Apache 2 license