Add support for passing Vault namespace

[rayokota]

This is useful when working with Vault Enterprise

[juergw]

Do you need to use the registry.KMSClient interface? If not and you only need an AEAD, then you can already do that if you use hcvault.NewAEAD:

like this:

client, err := api.NewClient(cfg)
if err != nil {
...
}
client.SetNamespace(namespace)
client.SetToken(token)
...
a, err := hcvault.NewAEAD("/transit/keys/key-1", client.Logical())
if err != nil {
...
}

This also has the advantage that it doesn't use the "context" request parameter to set the associated data, but uses the "associated_data". If you want to use "context" to be compatible with the previous behavior, then you may need to pass the hcvault.WithLegacyContextParamater() option.

[rayokota]

Yes, I need the registry.KMSClient interface

[juergw]

I've added the function hcvault.NewClientWithAEADOptions here:
57c4fd3

I think that should cover your use-case, is that correct?