Hardcode list of known checksums to avoid network requests in most cases

[Marcono1234]

This goes in the same direction as #96; was also suggested in #57 (comment)

What do you think about hardcoding the list of all known Gradle wrapper checksums into this action? Then during validation, the action first checks if the checksum is in that hardcoded list, or is specified in allow-checksums. And only if both checks fail it sends network requests to fetch the checksums.

This would hopefully have the following advantages:

• Action is more efficient and reduces network traffic for Gradle site
• Fewer spurious action failures due to network issues

It would not be an issue if the hardcoded list of checksums becomes outdated (possibly by months or even years); it probably still provides an advantage for the majority of the users, assuming they use older wrapper versions.

Alternative

Tell users to use the allow-checksums input. However, that risks reducing security because it might then become a usual action to update the allow-checksums input when updating the wrapper, and maintainers might then not properly verify that the allow-checksums is actually one of the officially listed ones. This allows a malicious user to simply specify the checksum of their malicious wrapper, and claim it is the official checksum, hoping the maintainer does not verify it.

[JLLeitschuh]

This is a solid idea, and absolutely worth doing. I poked at this a bit this afternoon, but I don't have the time right now to fully implement it.

[JLLeitschuh]

@big-guy @bigdaz thoughts? Would you accept a pull request that added this?

[bigdaz]

I think it's a good idea. I always thought it was a bit brain-dead that the action fetched all of the checksums every time.
An alternative implementation could be to use GitHub Actions caching to save the values that have been fetched dynamically.

[Marcono1234]

@JLLeitschuh, I have created an initial implementation for this in #167. I hope that is ok; but if you were already working on a solution for this, feel free to close my PR, and optionally use parts from it if you want.