Support optional/configurable IAMEndpoint for Minio Client (#32581)

[mowoc-ocp]

Targeting issue #32271

This modification allows native Kubernetes + AWS (EKS) authentication with the Minio client, to Amazon S3 using the IRSA role assigned to a Service account by replacing the hard coded reference to the DefaultIAMRoleEndpoint with an optional configurable endpoint.

Internally, Minio's credentials.IAM provider implements a discovery flow for IAM Endpoints if it is not set.

For backwards compatibility:

• We have added a configuration mechanism for an IamEndpoint to retain the unit test safety in minio_test.go.
• We believe existing clients will continue to function the same without needing to provide a new config property since the internals of Minio client also often resolve to the http://169.254.169.254 default endpoint that was being hard coded before

To test, we were able to build a docker image from source and, observe it choosing the expected IAM endpoint, and see files uploaded via the client.

[wxiaoguang]

by the way, "app.example.ini" and related documents https://gitea.com/gitea/docs/pulls also need to update 🙏

Wait for config example update before merge. If there is any difficulty, maintainers could also help.

[mowoc-ocp]

@wxiaoguang thank you for the feedback, I'll take care of the documentation PR today!

[mowoc-ocp]

Added the requested updates for app.example.ini and opened https://gitea.com/gitea/docs/pulls/103

Let me know if there is any feedback on documentation structure or anywhere I missed that I also need to update. Thanks!