Add peek functionality to index reader and use it in the file source (2.8.x)

[brycemcanally]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.85%. Comparing base (a90d866) to head (d9dfabd).

Additional details and impacted files

@@            Coverage Diff             @@
##            2.8.x    #9961      +/-   ##
==========================================
+ Coverage   49.72%   49.85%   +0.13%     
==========================================
  Files         565      565              
  Lines       69254    69270      +16     
==========================================
+ Hits        34435    34537     +102     
+ Misses      31353    31302      -51     
+ Partials     3466     3431      -35     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[FahadBSyed]

This looks intentional, but just to confirm: Do we only want to peek if the next file index is past the range?
Otherwise, it looks like range indices will continue to be read until we get to the next file index.

I assume this is needed so we emit the peeked file, maybe that's something worth calling out with a comment so it doesn't have to be inferred and we're aware of that later if we need to modify this code.

[brycemcanally]

Yeah, we want to check the first file index after the end of the path range. This may involve traversing additional range indexes if the end of the path range lines up with a chunk boundary.