memory usage estimator

[jshook]

This is a utility that we can use to get an empirical estimation of how much heap a given index or indexing process will take. It is a part of a larger strategy for managing heap usage safely.

Why? Here are the key reasons:

• We can take an index config and model "what if" heap usage very accurately, to understand how many vectors can reasonably fit on a system ahead of time.
• It is meant to provide advisory limits which we can use as a reliability gate without locking in bad administrative limits which do not size appropriately when things change
• This is a piece of a bigger idea to robustify heap usage.
• You do this once per index and you have a cost model for it without further tracking, counting work.

[github-actions]

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[MarkWolters]

Nit: We generally default to 1.2 for overflowRatio

[jshook]

noted, I'll follow this PR up with an improved one.

[MarkWolters]

Adds some useful functionality