credentials: use impl Into<String>

[paolobarbolini]

No description provided.

[codecov]

Codecov Report

Merging #41 (f851adc) into main (bb4ff05) will not change coverage.
The diff coverage is 91.66%.

@@           Coverage Diff           @@
##             main      #41   +/-   ##
=======================================
  Coverage   91.53%   91.53%           
=======================================
  Files          26       26           
  Lines        1300     1300           
=======================================
  Hits         1190     1190           
  Misses        110      110           

Impacted Files	Coverage Δ
src/credentials/serde.rs	71.05% <0.00%> (ø)
src/credentials/mod.rs	97.72% <100.00%> (ø)
src/credentials/rotating.rs	95.58% <100.00%> (ø)

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[guerinoni]

LGTM