Refactor: Rename BlobReader to BlobFinder

[olaotesile]

Renamed BlobReader and BlobWriter to BlobFinder as suggested by @janwas in TODO. Also Updated associated files, tests, and CMakeLists.txt.

Addresses the TODO in io/blob_store.h by renaming the classes and files to BlobFinder/BlobWriter for better clarity. Verified file renames and updated CMake configuration.

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[jan-wassenberg]

Hello and happy holidays :) Thanks for tackling this. Would you mind retargeting this toward the dev branch? We are unable to merge into main directly.
Also, are you able to sign the CLA, which is also required before merging? https://cla.developers.google.com/

[olaotesile]

Hi Jan,
Happy holidays to you too!

I've retargeted this to the dev branch, signed the CLA, and merged the latest dev changes to resolve the conflicts. Everything should be ready for review now.

I’m planning to focus heavily on low-level optimization and ML infrastructure this year, so I'll be keeping an eye on the issue tracker for more ways to contribute to gemma.cpp. Thanks!