add STS support for `mc admin` command

Community Contribution License

All community contributions in this pull request are licensed to the project maintainers under the terms of the [Apache 2 license] (https://www.apache.org/licenses/LICENSE-2.0). By creating this pull request I represent that I have the right to license the contributions to the project maintainers under the Apache 2 license.

Description

add STS support for mc admin commands

Motivation and Context

To allow invoke subcommands in the admin section such as user, group, policy, etc.

Example:

MC_HOST_demomc=http://localhost:9000  \
MC_STS_ENDPOINT=https://sts.minio-operator.svc.cluster.local:4223/sts/ns-1 \
MC_WEB_IDENTITY_TOKEN_FILE=/var/run/secrets/kubernetes.io/serviceaccount/token \
./mc admin policy create demomc bucket-rw policy.json

How to test this PR?

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Optimization (provides speedup with no functional changes)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

[ ] Fixes a regression (If yes, please add commit-id or PR # here)
[x] Unit tests added/updated
[ ] Internal documentation updated
[ ] Create a documentation update request here

minio / mc