Add 'cloud-terminal' (workspace) icon to icon library

Purpose

Related to #356 (closed), add new 'cloud-terminal' icon to represent "workspace".

Checklist

For a new icon and/or illustration:

• Link to related design issue: #356 (closed)
• If this adds an icon ensure that it follows the new icon checklists.
• If this adds an illustration ensure that it follows the new illustration checklists.

For third-party trademarks:

• If this adds or updates an icon or illustration that includes a third-party trademark, ensure that it follows the process for adding third-party trademarks to GitLab.

Closes #356 (closed)