feat(blueprints): add possibility enable/disable default brand

[DrummyFloyd]

Details

Because ovveriding default brand is bit painful with docker/kubernetes

=> mount entpy file to replace default-brand.yaml

can we instead add a env value like this,
will not breaking existing due to default fallback, but can be easily un defaulted what do you thiink ?

NB: this approch is IaC friendly

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Checklist

None of them has been made du the minor change
not applicable i guess

• Local tests pass (ak test authentik/)
• The code has been formatted (make lint-fix)

If an API change has been made

• The API schema has been updated (make gen-build)

If changes to the frontend have been made

• The code has been formatted (make web)

If applicable
NB: will be done if accepted

• The documentation has been updated
• The documentation has been formatted (make website)

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