Issue 6823 - Add caching of HEAD requests objectmeta to readahead store

[m09526]

Make sure you have checked all steps below.

Issue

• My PR fully resolves the following issues. I've referenced an issue in the PR title, for example "Issue 1234 - My
  Feature". Note that before an issue is finished, you can still make a pull request by raising a separate issue
  for your progress.
  • Resolves Add caching of HEAD requests ObjectMeta to readahead store #6823

Tests

• My PR adds the following tests based on our test strategy OR does not need testing for this extremely good reason:
  -rust/objectstore_ext/src/readahead.rs

Documentation

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  separate issue for that below.
• If I have added new Java code, I have added Javadoc that explains it following our conventions and style.
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