Temporary access tokens for minio, for outside Kubeflow

[blairdrummond]

A use case encountered by PowerBI users is the need to get S3 Access to their buckets
from an external environment. They can still log into kubeflow to get tokens from Vault,
but this is a long process, and things will probably be more secure if it's possible to get
a temporary token issued for these ad-hoc uses.

CC @zachomedia

[justbert]

They will be able to do this via the VaultUI once the OIDC access to Vault is setup.

[blairdrummond]

This is done, but needs to be added to the docs.

[blairdrummond]

Oh I have that in #149