validateSignedMessage bug fix when signed in Korean message

[nohkwak]

Proposed changes

• When Korean text message is signed, the signature is not properly verified with the validateSignedMessage function.
• Add logic to change encoding with utf8ToHex function if it is not Hex encoding.

Types of changes

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• Bugfix
• New feature or enhancement
• Others

Checklist

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• I have read the CONTRIBUTING GUIDELINES doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Related issues

• https://krustuniverse.atlassian.net/browse/CDAP-36

Further comments

• none