Allow usage of namespaces in Hashicorp vault encryption plugin

Currently there seems to be no way to use namespaces in hashicorp vault which are nearly always used in an enterprise environment where hashicorp vault is setup and utilized.
See: https://developer.hashicorp.com/vault/docs/enterprise/namespaces

Without namespace support as well as without proper authentication MDEV-29239 the plugin isn't ready yet for companies that want to use it cause they already have an enterprise vault.

Especially namespaces hurt since they are normally mandatory in an enterprise field to separate KV stores.