move sign method to utils

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.50%. Comparing base (521f688) to head (743d477).

Files	Patch %	Lines
sdk/utils/utils.go	91.30%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #636 +/- ## ========================================== - Coverage 93.52% 93.50% -0.03% ========================================== Files 48 48 Lines 2517 2539 +22 ========================================== + Hits 2354 2374 +20 - Misses 106 107 +1 - Partials 57 58 +1 ```

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