[Need-Discussion] Separate tier add command to sub commands per tier backend

[vadmeste]

'mc ilm tier edit' is a bit weird; it seems we need to do a revision for edit command in mc and MinIO server API as well

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Description

Currently the flags help is crowded. This commit will split the add
command to more sub-commands, minio, s3, gcs, azure; so it is easy
for users to see associated flags for each backend tier

Motivation and Context

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

[klauspost]

Solid improvement 👍🏼 from me.

[harshavardhana]

@vadmeste what discussion is pending?

[klauspost]

--- FAIL: TestAutoCompletionCompletness (0.00s)
    auto-complete_test.go:56: Missing completion function: Completion for `/ilm/tier/add/azure` not found

[vadmeste]

@harshavardhana we need to fix 'mc ilm tier edit' command. Currently there is no easy change without breaking the API, if you take a look at that command help, you will understand we cannot have subcommands in it. It looks like this requires a breaking API in the server side as well.