What does this PR do? Please describe:
This PR adds a file integrity check for tokenizers, models, and datasets to ensure they were downloaded properly with a SHA-1 checksum. If the checksum listed on the asset card doesn't match, the download_manager will raise an AssetDownloadError. If there is no listed checksum on the asset card, the download_manager will proceed like normal but make a call to log.warning about the missing checksum field.

Fixes #839

Does your PR introduce any breaking changes? If yes, please list them:
Adds an additional checksum parameter to the download_manager's methods for downloading tokenizers, models, and datasets.

Check list:

• Was the content of this PR discussed and approved via a GitHub issue? (no need for typos or documentation improvements)
• Did you read the contributor guideline?
• Did you make sure that your PR does only one thing instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (no need for typos, documentation, or minor internal changes)