refactor(common): builder::LinkedChunkBuilder::* becomes lazy_loader::*

[Hywan]

This patch renames the builder module to lazy_loader. The LinkedChunkBuilder's methods are now functions. The LinkedChunkBuilder struct is removed. Finally, LinkedChunkBuilderError is renamed LazyLoaderError.

The LinkedChunkBuilderTest struct is kept for the moment. It's going to be replaced soon. Edit: see #4695.

Easier to review without whitespace in the diffs.

---

• Address [meta] EventCache storage #3280
• Follow up of feat(sdk): Implement EventCache lazy-loading #4632

[codecov]

Codecov Report

Attention: Patch coverage is 86.76471% with 9 lines in your changes missing coverage. Please review.

Project coverage is 85.90%. Comparing base (2eb2ae7) to head (d8f4f63).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
.../matrix-sdk-common/src/linked_chunk/lazy_loader.rs	85.93%	9 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4686   +/-   ##
=======================================
  Coverage   85.89%   85.90%           
=======================================
  Files         292      292           
  Lines       33912    33909    -3     
=======================================
  Hits        29129    29129           
+ Misses       4783     4780    -3     

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