Allow customization of the size of persistent storage that the GitLab workspace uses per pod
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### Proposal
Currently, the remote workspace storage size for a pod is [hardcoded at 15GB](https://gitlab.com/gitlab-org/gitlab/-/blob/84a93caf554ebcdc8aaa853c5382e8bd85e6a071/ee/lib/remote_development/workspaces/create/volume_component_injector.rb#L22) and the proposed is to allow configuration of this value by the customer.
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