[JENKINS-74836] Allow using a file based ssh credential via system property

[mikecirioli]

JENKINS-74836

For some use cases it may not be desirable to create a Jenkins ssh credential to use for connecting to agents, as this credential may be visible to users in the ux, or you may find it easier to manage this private key as a mounted k8s secret.

To enable this functionality simply define a new System.property as follows:
SSH_KEY_PAIR_PRIVATE_KEY_FILE=/path/to/plaintext/private_key.pem

If this system property is defined then any credential configured in the cloud config page will be ignored.

configuration validation has also been updated to support the new feature:

file based key, no jenkins credentials - form validation

`file based key, no jenkins credentials - test connection

file based key, jenkins credentials selected - form validation

file based key, jenkins credentials selected - test connection

Testing done

• Manually tested and works as expected in my k8s test cluster.
• fixup 1 failing unit test
• unit test for new functionality

Submitter checklist

• 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
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[mikecirioli]

feedback addressed @jglick , i am fixing up the testing now

[mikecirioli]

@fcojfernandez i believe this is ready for your review

[amuniz]

LGTM

[fcojfernandez]

would it be possible to have more tests? Something to test what prevails if the user includes the credentials but the system property is overriding the value

[mikecirioli]

added screenshot and validated manually again as awell @fcojfernandez