Add symbols for SCM traits

[jonesbusy]

Hi,

Add symbols to avoid using configure block when configuring those traits via JobDSL

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did

Regards,

[codecov]

Codecov Report

Merging #289 (5dcd5bc) into master (1f6579c) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #289   +/-   ##
=========================================
  Coverage     80.79%   80.79%           
  Complexity      170      170           
=========================================
  Files            16       16           
  Lines           526      526           
  Branches         49       49           
=========================================
  Hits            425      425           
  Misses           79       79           
  Partials         22       22           

Impacted Files	Coverage Δ
...ecks/github/config/GitHubSCMSourceChecksTrait.java	22.22% <ø> (ø)
...ithub/status/GitHubSCMSourceStatusChecksTrait.java	75.00% <ø> (ø)

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