Use temporary oauth2 tokens for Google registries

[DennisD2]

Pull Request Description

Describe what this PR does / why we need it

For Google registries (*.gcr.io), an OAUTH2 token can be created from a (base64 encoded) service account token.
Define credentials like this:

username: _oauth2_
password: <base64 encoded permanent service account token>

Does this pull request fix one issue?

Fixes #62

Describe how you did it

Added method for getting the temporary OAUTH2 token from the permanent service account token

Describe how to verify it

Define a google repo (with gcr.io in hostname), with credentials including a permanent service account token as password value, use the string _oauth2_ as username, and use this as image sync target. The sync will work, and use an on-the-fly created OAUTH2 token

Special notes for reviews

[codecov-commenter]

Codecov Report

Merging #93 (501b042) into master (8a981fe) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #93   +/-   ##
=======================================
  Coverage   53.96%   53.96%           
=======================================
  Files           1        1           
  Lines          63       63           
=======================================
  Hits           34       34           
  Misses         24       24           
  Partials        5        5           

Flag	Coverage Δ
unittests	53.96% <ø> (ø)

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[hhyasdf]

@DennisD2 thanks, we are almost done here! Could you please fix the golangci-lint error?

[DennisD2]

Sorry was some days off. Will check the lint issue now.

[DennisD2]

All green now :-)

[hhyasdf]

Thanks for your contribution!