Change the way to calculate content md5 in localUnderFileSystem

[Jackson-Wang-7]

What changes are proposed in this pull request?

change the way to calculate the content hash of files from localUnderFileSystem to get the md5 of content.

Why are the changes needed?

It's better to compare the md5 of the file by using hexadecimal format instead of Base64 format.

Does this PR introduce any user facing changes?

no

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[jja725]

Different storage has different encoding(ex. s3 has Hex while gcs has base64), so I recommend we change the content hash to a proto CheckSum('AlgorithmName', 'bytes') so we can compare if necessary. I can draft the code if you guys are OK with that @Jackson-Wang-7 @elega