Introduce Runtime associated type for Load,Loadable in diskann-record

[Suhas Jayaram Subramanya (suhasjs)]

• Does this PR have a descriptive title that could go in our release notes?
• Does this PR add any new dependencies?
• Does this PR modify any existing APIs?
• Is the change to the API backwards compatible?
• Should this result in any changes to our documentation, either updating existing docs or adding new ones?

This (draft) PR addresses the problem of providing some runtime state/config/parameters during index deserialization.

One obvious example of what this enables: an inmem-2.0 index is constructed with max_slots = 1 million and contains 900k active points. After serialization to disk, only the data in the active slots is persisted, so using a InMemIndexRuntime parameter as an arg to the Provider's Load impl with max_slots = 2 million to support a larger index size than originally intended.

Another obvious use-case involves setting scratch spaces and number of threads, etc.

Reference Issues/PRs

#1386

What does this implement/fix? Briefly explain your changes.

Any other comments?

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.43%. Comparing base (2e1fc7e) to head (dabc89e).

Additional details and impacted files

@@ Coverage Diff @@## main #1387 +/- ##
=======================================
Coverage 91.43% 91.43% =======================================
Files 522 522 Lines 100766 100766 =======================================
Hits 92134 92134 Misses 8632 8632 

Flag	Coverage Δ
miri	91.43% <ø> (ø)
unittests	91.23% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.
see 3 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.