Add timja to credentials plugin

[timja]

Link to GitHub repository

see jenkinsci/credentials-plugin#984 (review)

Jan and I plan to do a UI overhaul on credentials plugin and being a maintainer of the plugin would help make it easier.

cc @jglick

When modifying release permission

List the GitHub usernames of the users who should have commit permissions below:

• @username1
• @username2

This is needed in order to cut releases of the plugin or component.

If you are modifying the release permission of your plugin or component, fill out the following checklist:

Release permission checklist (for submitters)

• The usernames of the users added to the "developers" section in the .yml file are the same the users use to log in to accounts.jenkins.io.
• All users added have logged in to Artifactory and Jira once.
• I have mentioned an existing team member of the plugin or component team to approve this request.

When enabling automated releases (cd: true)

Follow the documentation to ensure, your pull request is set up properly. Don't merge it yet.
In case of changes requested by the hosting team, an open PR facilitates future reviews, without derailing work across multiple PRs.

Link to the PR enabling CD in your plugin

CD checklist (for submitters)

• I have provided a link to the pull request in my plugin, which enables CD according to the documentation.

Reviewer checklist

• Check that the $pluginId Developers team has Admin permissions while granting the access.
• In the case of plugin adoption, ensure that the Jenkins Jira default assignee is either removed or changed to the new maintainer.
• If security contacts are changed (this includes add/remove), ping the security officer (currently @Wadeck) in this pull request. If an email contact is changed, wait for approval from the security officer.

There are IRC Bot commands for it.