Vault Namespaces [0] is a feature available in Vault Enterprise that
can be considered as a more advanced isolation feature on top of current
KV Mountpoint option in Castellan Vault plugin.
Passing a namespace in all request headers (including Auth) allows to organize
Vault-in-Vault style of isolation, with clients using the same simple URI path
but accessing separate sets of entities in Vault.
[0] https://www.vaultproject.io/docs/enterprise/namespaces
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