cryptutil: use bytes for hmac

[calebdoxsey]

Summary

Update the HMAC methods in cryptutil to takes []byte instead of string. Also update the signed URL to take in []byte.

Checklist

• reference any related issues
• updated docs
• updated unit tests
• updated UPGRADING.md
• add appropriate tag (improvement / bug / etc)
• ready for review

[codeclimate]

Code Climate has analyzed commit b3b4004 and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #2067 (b3b4004) into master (9de340b) will increase coverage by 0.0%.
The diff coverage is 80.0%.

@@          Coverage Diff           @@
##           master   #2067   +/-   ##
======================================
  Coverage    59.4%   59.4%           
======================================
  Files         167     167           
  Lines       11233   11278   +45     
======================================
+ Hits         6675    6707   +32     
- Misses       3780    3793   +13     
  Partials      778     778           

Impacted Files	Coverage Δ
internal/middleware/middleware.go	100.0% <ø> (ø)
internal/urlutil/signed.go	92.7% <ø> (ø)
authenticate/middleware.go	54.5% <50.0%> (ø)
authenticate/handlers.go	82.4% <66.6%> (ø)
authorize/check_response.go	81.6% <100.0%> (ø)
internal/httputil/reproxy/reproxy.go	77.1% <100.0%> (ø)
pkg/cryptutil/hmac.go	100.0% <100.0%> (ø)
proxy/state.go	84.6% <100.0%> (ø)
internal/databroker/config_source.go	51.1% <0.0%> (-4.9%)	⬇️
pkg/storage/inmemory/backend.go	86.7% <0.0%> (-3.7%)	⬇️
... and 4 more