add STS support for mc admin command

[pjuarezd]

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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)
• 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)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here

[ravindk89]

@pjuarezd is this only for the Operator STS loop? Or could this also be used for non-Operator STS, e.g. OIDC or AD/LDAP loops?