feat(vault-token): add optional vault enterprise namespace variable (#108)

Added an optional envvar to vault-token module to handle communicating
with a non default vault namespace.

in vault enterprise, you can run multiple secure isolated vault
environments from the one vault server.
each namespace has it's own authentication methods and secrets engines. 
vault uses the VAULT_NAMESPACE envvar to determine the namespace to use.
no value, or either `root` or `/` will use the root (default) namespace,
any other value will use a different namespace

in vault community edition, the only supported namespace is "root", no
other namespaces can be used.

in HCP vault dedicated (the saas hosted version), you cant access vault
without a namespace set

this defaults to not setting the env var, so is backwards compatible,
and works with vault CE

---------

Co-authored-by: Birdie K <5210502+moo-im-a-cow@users.noreply.github.com>

Author: moo-im-a-cow

registry/coder/modules/vault-token/README.md

@@ -20,11 +20,12 @@ variable "vault_token" {
 }
 
 module "vault" {
-  source      = "registry.coder.com/coder/vault-token/coder"
-  version     = "1.1.0"
-  agent_id    = coder_agent.example.id
-  vault_token = var.token # optional
-  vault_addr  = "https://vault.example.com"
+  source          = "registry.coder.com/coder/vault-token/coder"
+  version         = "1.2.0"
+  agent_id        = coder_agent.example.id
+  vault_token     = var.token # optional
+  vault_addr      = "https://vault.example.com"
+  vault_namespace = "prod" # optional, vault enterprise only
 }
 ```
 
@@ -74,7 +75,7 @@ variable "vault_token" {
 
 module "vault" {
   source            = "registry.coder.com/coder/vault-token/coder"
-  version           = "1.1.0"
+  version           = "1.2.0"
   agent_id          = coder_agent.example.id
   vault_addr        = "https://vault.example.com"
   vault_token       = var.token

registry/coder/modules/vault-token/main.tf

@@ -26,6 +26,11 @@ variable "vault_token" {
   sensitive   = true
   default     = null
 }
+variable "vault_namespace" {
+  type        = string
+  description = "The Vault namespace to use."
+  default     = null
+}
 
 variable "vault_cli_version" {
   type        = string
@@ -62,3 +67,10 @@ resource "coder_env" "vault_token" {
   name     = "VAULT_TOKEN"
   value    = var.vault_token
 }
+
+resource "coder_env" "vault_namespace" {
+  count    = var.vault_namespace != null ? 1 : 0
+  agent_id = var.agent_id
+  name     = "VAULT_NAMESPACE"
+  value    = var.vault_namespace
+}