feat: allow retaining parity SLA to be configurable

[harshavardhana]

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Description

feat: allow retaining parity SLA to be configurable

Motivation and Context

At scale, customers might start with failed drives, causing
skew in the overall usage ratio per EC set.

Make this configurable so customers can turn it off as
needed depending on how comfortable they are with the
SLA.

How to test this PR?

To turn off retaining parity SLA and optimize for capacity
for objects when drives have failed, do this only when you
know what you are doing and usable space is more important
to you than additional parity for objects.

mc admin config set alias/ storage_class optimize=capacity

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]

LGTM