Add UTF-8 support for Flask jsonify

[caozhuozi]

What this PR does / why we need it:

Make Flask jsonify encoding configurable, that is, add UTF-8 support for Flask jsonify

Which issue(s) this PR fixes:

Fixes #4261

Special notes for your reviewer:

[seldondev]

Hi @caozhuozi. Thanks for your PR.

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[seldondev]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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To complete the pull request process, please assign jklaise
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[caozhuozi]

/assign @jklaise

[axsaucedo]

Thank you for the contribution @caozhuozi 👍

/test integration

[axsaucedo]

/test integration

[axsaucedo]

NIce one - thank you for the contribution @caozhuozi !