feat: expose the credential username via an optional usernameVariable - #285

The sshagent step only exposed SSH_AUTH_SOCK and SSH_AGENT_PID, so there was no way to pass the credential's SSH user to ssh -l without hardcoding it (JENKINS-45312).

This adds an optional usernameVariable parameter that binds the first resolved credential's username to that environment variable inside the block:

sshagent(credentials: ['my-key'], usernameVariable: 'SSH_USER') {
  sh 'ssh -l $SSH_USER example.org uname -a'
}

It mirrors the withCredentials usernameVariable convention, survives agent resume (it is stored on the step execution rather than the transient step), and is unset by default so existing pipelines are unaffected. With multiple credentials it uses the first.

Testing

Added exposesCredentialUsernameViaUsernameVariable to SSHAgentStepWorkflowTest, which runs sshagent(..., usernameVariable: 'SSH_USER') and asserts $SSH_USER resolves to the credential's username. mvn test -Dtest=SSHAgentStepWorkflowTest#exposesCredentialUsernameViaUsernameVariable passes.

Closes #234