Set Managed Namespace in Vault Config File

[lucymhdavies]

Is your feature request related to a problem? Please describe.
#10588 looks to be a useful feature, and I suspect I've seen it in practice in my personal single-node HCP Vault:

However, to enable this requires setting a feature flag with sys/internal/ui/feature-flags.

This (presumably) would apply to the entire cluster including downstream enterprise performance replica clusters.

Additionally, that PR does not appear to add any documentation, and I could not find any on the https://www.vaultproject.io/ site.

Describe the solution you'd like
In our use-case, we have a performance replica cluster, to which we only replicate one namespace.
So this "automatically set namespace via feature flag" seems super useful, but the limitation that it would seemingly apply to all (replicated) clusters makes the current implementation not ideal.

Being able to set this in the Vault config file instead would allow us to set this flag only for our performance replica cluster.

Describe alternatives you've considered
For now, the URL we give to the users of this replica vault is something like:
https://child-namespace.vault.fancycorp.io/ui/vault/auth?namespace=child%namespace
This will pre-populate the namespace textbox.

But a solution where we just give our users the URL https://child-namespace.vault.fancycorp.io would be better.