[Core Feature] Further templatize user namespaces

[katrogan]

Motivation: Why do you think this is important?
The flyteadmin config currently allows users to define user namespaces in which flyte workflows run and dynamically modify them (assign resource quotas, add role bindings, image pull secrets to default/custom service accounts, etc) using the following formats

• <project>-<domain>
• <project>
• <domain>

we should introduce templating to this config-specified attribute to make it further customizable, e.g. my_fixed_prefix-<project>-<domain

Goal: What should the final outcome look like, ideally?
It should be easy to modify permitted user namespaces in which flyte workflows run under.

Describe alternatives you've considered
Current implementation

[Optional] Propose: Link/Inline OR Additional context
If you have ideas about the implementation please propose the change. If inline keep it short, if larger then you link to an external document.

[katrogan]

cc @wild-endeavor