Add shared provider core

[j0rd1s3rr4n0]

Summary

• Adds a shared fileuploader provider core with normalized provider metadata, upload results, and provider errors.
• Registers GoFile, AnonFilesNew, JSONBin, AnonFiles, and BayFiles behind a common interface.
• Keeps deprecated providers available while surfacing clear normalized errors.
• Adds provider-core unit tests with mocked HTTP clients and no network dependency.

Validation

• python -m unittest tests.test_provider_core tests.test_gofile_encryption tests.test_jsonbin_uploader
• python -m py_compile fileuploader\__init__.py fileuploader\models.py fileuploader\core.py fileuploader\registry.py fileuploader\providers.py tests\test_provider_core.py

Closes #10.