[jenkins] Improvements to backend config

[iamEAP]

Hey, I just made a Pull Request!

• Removes the call to config.getConfig('jenkins'), which unnecessarily throws if the jenkins key is not set.

• Adds conditional logic to the config schema to make it clearer at build-time how the keys under jenkins should be provided (rather than having to wait until run-time to find out), e.g.

  yarn backstage-cli config:check @backstage-community/plugin-jenkins-backend --strict
  

  Will now show an error in situations like:

  • Only some of apiKey, username, and baseUrl are provided
  • Any of the above are provided, but an instances array is also provided

Closes #2858

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Tests for new functionality and regression tests for bug fixes
• All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage-community/plugin-jenkins-backend	workspaces/jenkins/plugins/jenkins-backend	patch	v0.11.0