Refactor NetworkHelper into a library

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Refactor NetworkHelper into a small library to be used across all UCSD Flutter projects.

Motivation NetworkHelper is being used across multiple UCSD projects, including Campus Mobile and SPM. The code is currently being duplicated. The version in SPM is more up to date compared to Campus Mobile. Dio version can be maintained automatically for all projects if it is updated globally through a separate package. Any bugfixes or new features can also automatically be distributed to all projects relying on this package.

Additional goals: Refactor code structure, improve API for better readability, optimize efficiency, and upgrade to a new version of Dio while we are doing Dart 3 migration.

Undecided: Should this library be stored in GitHub, BitBucket, or somewhere else? Do we want to keep it as a subproject of Campus Mobile for now?

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Created a private repository containing the NetworkHelper library. Need to add documentation to it, then we can publish it and add it to as a dependency to Campus Mobile's pubspec.yaml