[flags] extract tabletenv seconds type

[ajm188]

Description

Two main goals:

1. Move the Seconds type out of package tabletenv, so that in a separate PR I have in progress, another package (which tabletenv will need to import) can import or otherwise reference the type without creating an import cycle.
2. Deprecate the "we will parse your float and turn it into seconds" behavior in favor of just using plain-old time.Duration types. One release after this ships, we would then be able to turn these types from our custom type into Durations and delete the custom type from the codebase.

For my purposes, in full transparency, I need (1), and only want (2) if it's not too 🌶️ of a move.

Some notes

tabletenv.ReplicationTrackerConfig

this one is a bit annoying because while the flags are themselves already time.Duration (where we want to end up), the config also can be marshalled from yaml, so we need to shim in a potentially breaking change (hence the deprecation phase) to read pure durations there

Related Issue(s)

• Resolves Move Seconds type from tabletenv to flagutil (or other appropriate package) #11891

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[rohit-nayak-ps]

lgtm

❤️ the DeprecatedFloat64Seconds pattern