Restructure adding and removing policies from users and groups

[taran-p]

Description

Changes mc admin policy add to mc admin policy create
Replaces mc admin policy (set|unset|update) commands with mc admin policy (attach|detach)
Adds mc admin policy entities

Motivation and Context

Implements changes requested in issues below, with some modifications requested offline.

resolves #4272
resolves #4273
resolves #4274

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

[harshavardhana]

This is a breaking change FWIW

[harshavardhana]

PTAL @donatello

[donatello]

@taran-p we are more or less ready to take this PR in with minor changes - please resolve the conflicts.

[donatello]

PTAL @harshavardhana @kannappanr this is ready to merge

This is a breaking change and we should note it in the release notes in the next release.