Unescape characters for string output type in the contract response

[vbaranov]

Closes #4445

Motivation

Escaping of \x blocks decoding of UTF-8 encoded string in contract string type output.

Changelog

Add unescape function taken from https://elixirforum.com/t/how-to-properly-convert-this-octal-encoded-utf16-be-string-to-utf8/33220/11 and implement it in the case of string output type.

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
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