Add support for digest_class config

[aduth]

This pull request proposes to add support for a digest_class configuration, to override the behavior of asset digesting.

Why?

• More control over how the digest is computed.
  • In our use-case, my interest is more in being able to shorten the length of the digest from 32 characters to 7 or 8 characters, and this is a roundabout way of doing that while offering greater overall control.
• Feature parity with the equivalent Sprockets configuration, to streamline the process of migrating from Sprockets to Propshaft.

Open Questions:

• Should this have any impact on the behavior of already_digested?, whose implementation may vary depending on the digest logic?
• Is there a better way to "pass down" configuration options than carrying along the constructor argument as proposed here?
  • I was largely following the prior art of how version was passed along
• If there's not an interest in adding this configuration, would there be more reception to either (a) allowing the digest length to be configuration, or (b) changing the default to a shorter length? Either of these would satisfy at least my own intended use-case.

[brenogazzola]

@aduth Thank you for the PR, however one of the reasons we've built this gem was to have a clean slate to work on compared to Sprockets. This means that feature parity with Sprockets is the opposite of what we want. We can revisit this at a later date, whoever as long was Sprockets is supported, features that are not core to Propshaft (full support for no build, better support for builders) are not our focus.

[aduth]

@brenogazzola Totally fair on the point of feature parity with Sprockets.

My interest was more in shortening the current digest length, and feature parity was a convenient approach to achieving this. I added some alternative approaches in the original comment; would there be any more of an appetite for these?

If there's not an interest in adding this configuration, would there be more reception to either (a) allowing the digest length to be configuration, or (b) changing the default to a shorter length?

Happy to create an issue if that's the best path forward for discussing.