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

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.

modules/setting/storage_test.go

@@ -470,6 +470,19 @@ MINIO_BASE_PATH = /prefix
 	cfg, err = NewConfigProviderFromData(`
 [storage]
 STORAGE_TYPE = minio
+MINIO_IAM_ENDPOINT = 127.0.0.1
+MINIO_USE_SSL = true
+MINIO_BASE_PATH = /prefix
+`)
+	assert.NoError(t, err)
+	assert.NoError(t, loadRepoArchiveFrom(cfg))
+	assert.EqualValues(t, "127.0.0.1", RepoArchive.Storage.MinioConfig.IamEndpoint)
+	assert.EqualValues(t, true, RepoArchive.Storage.MinioConfig.UseSSL)
+	assert.EqualValues(t, "/prefix/repo-archive/", RepoArchive.Storage.MinioConfig.BasePath)
+
+	cfg, err = NewConfigProviderFromData(`
+[storage]
+STORAGE_TYPE = minio
 MINIO_ACCESS_KEY_ID = my_access_key
 MINIO_SECRET_ACCESS_KEY = my_secret_key
 MINIO_USE_SSL = true