Adjust the length of blob cache docs for Lucene metadata files #69692
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Today searchable snapshots IndexInput implementations use the
blob store cache to cache the first 4096 bytes of every Lucene files.
After some experiments we think that we could adjust the length of
the cached data depending of the Lucene file that is read, caching
up to 64KB for Lucene metadata files (ie files that are fully read
when a Directory is opened) and only 1KB for other files.
The files that are cached up to 64KB are the following extensions:
The 64KB limit can be configured on a per index basis through a new
index setting. This change is extracted from #69283 and does not
address the caching of CFS files.
Backport of #69431