Fix: add missing sshUserPrivateKey options of CredentialsBinding Plugin for wrapper

[Aju100]

Added the options for SSHUserPrivateKey to the DSL plugin.

Problem:

• SSHUserPrivateKey is missing at the API reference of the job dsl plugin which is essentially required to automate to setup of this option in a secret file or key at the configuration of Jenkins jobs/pipeline.

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- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [ ] Link to relevant issues in GitHub or Jira
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- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue