Honor duration_seconds in assume-role profiles

[afarber]

Motivation and Context

Fixes #5695.

Assume-role profiles could specify duration_seconds, but the STS request did not use the configured value. STS therefore issued sessions using its default duration.

Modifications

• Map duration_seconds from assume-role profiles to AssumeRoleRequest.durationSeconds.
• Extract package-visible request construction for direct unit testing.
• Add regression tests for configured and absent duration values.
• Add a changelog entry.

Testing

./mvnw -pl :sts -am \
  -Dtest=AssumeRoleProfileTest \
  -Dsurefire.failIfNoSpecifiedTests=false test

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 (https://github.com/aws/aws-sdk-java-v2/blob/master/CONTRIBUTING.md) 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 added tests to cover my changes
• All new and existing tests passed
• I have added a changelog entry.
• My change is to implement 1.11 parity feature and I have updated LaunchChangelog
  (https://github.com/aws/aws-sdk-java-v2/blob/master/docs/LaunchChangelog.md)

License

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