Issue 214: CredentialsPlugin - support more than username/password

[kmanning]

CredentialsPlugin currently only supports username/password, but there are many more. Support all types of credential bindings.

Merge Checklist

• Have you linked this Pull Request to an Issue?
• Have you updated any relevant README files, to explain how this new feature works (with examples)?
• Have you added relevant test cases for your change?
• Do all tests and code style checks pass with ./gradlew check --info?
• Have you successfully run your change in a pipeline in a Jenkins instance?
• Have you updated the CHANGELOG?

[codecov-commenter]

Codecov Report

Merging #403 (a99eca9) into master (110d929) will increase coverage by 0.23%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #403      +/-   ##
==========================================
+ Coverage   82.06%   82.30%   +0.23%     
==========================================
  Files          53       53              
  Lines        1455     1452       -3     
  Branches      307      307              
==========================================
+ Hits         1194     1195       +1     
+ Misses        119      115       -4     
  Partials      142      142              

Impacted Files	Coverage Δ
src/CredentialsPlugin.groovy	73.07% <88.88%> (+11.00%)	⬆️

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[codezninja]

The change work as expected. Looks great.