Custom User Agent

[piksel]

Set a custom User-Agent header in the http client requests.
The value would be Watchtower/1.3.0 (with the appropriate version of course).

Fixes #986

[codecov]

Codecov Report

Merging #990 (39ccc91) into main (4e7b1e7) will increase coverage by 1.68%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #990      +/-   ##
==========================================
+ Coverage   54.47%   56.15%   +1.68%     
==========================================
  Files          25       25              
  Lines        1452     1453       +1     
==========================================
+ Hits          791      816      +25     
+ Misses        594      567      -27     
- Partials       67       70       +3     

Impacted Files	Coverage Δ
pkg/registry/digest/digest.go	42.37% <100.00%> (+42.37%)	⬆️

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