Add HashTo/GetCurrentHashAs{UInt32/64} methods

[stephentoub]

Fixes #76279

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[stephentoub]

@bartonjs, mind taking a look? Thanks.

[bartonjs]

Code LGTM. Based on the conversation I had with @GrabYourPitchforks when the API was proposed, and our assertion of GetCurrentHashCode() being king, I'd expect the endianness comments in all of the integer-based values (as a remarks remark or just as part of the returns ("Returns the [whatever algorithm] hash code using little-endian byte ordering."), maybe with a <seealso cref="GetCurrentHash()" />.

[stephentoub]

Thanks for the review.

[uweigand]

Hi @stephentoub , the new test cases added here all fail on s390x, from the logs it looks like an obvious endian problem:
https://helixre107v0xdeko0k025g8.blob.core.windows.net/dotnet-runtime-refs-heads-main-5428a61833d54fffa2/System.IO.Hashing.Tests/1/console.0c4ec733.log?helixlogtype=result

    System.IO.Hashing.Tests.Crc64Tests.VerifyHashToUInt64(testCase: One) [FAIL]
      Assert.Equal() Failure
                ↓ (pos 0)
      Expected: 42F0E1EBA9EA3693
      Actual:   9336EAA9EBE1F042
                ↑ (pos 0)

etc.