refactor(fs): use builder for initialization of SequentialFileReader

[kskalski]

Problem

We need a more advanced way to create io_uring sequential file reader with several tuning options.
The nearest uses for this will be:

• ability to enable/disable DIRECT_IO (we want to reserve ability to turn it off in case of bugs)
• ability to enable/disable registering buffer with kernel as FIXED (it requires memlock ulimit that is not always available)
• providing shared sqpoll FD
• adjusting IO kernel workers, sqsize, etc. -> those can be tuned depending on whether sqpoll is used or if we use many readers across threads or pushing lots of ops into single threaded one, etc.

All those low level options would clutter the API when required to be passed as parameters to constructor functions.

Summary of Changes

• add SequentialFileReaderBuilder, which uses defaults when created by new() and allows adjusting parameters and finally building the reader
• extract some blocks of code used for initialization into separate functions
• extract starting of reading into separate function (in the future we will allow building reader without providing the path, so start of reading will in fact be a separate flow)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.64865% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.5%. Comparing base (e9052f0) to head (0537ccd).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9420   +/-   ##
=======================================
  Coverage    82.5%    82.5%           
=======================================
  Files         895      895           
  Lines      322338   322361   +23     
=======================================
+ Hits       266042   266097   +55     
+ Misses      56296    56264   -32     

🚀 New features to boost your workflow:

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

[brooksprumo]