allow overriding agent's coder host/access url

```terraform
container {
name = "workspace"
image = local.container_image
image_pull_policy = "Always"
command = ["sh", "-c", replace(coder_agent.bird.init_script, "coder.tree", "coder.coder.svc.cluster.local")]
}
```

currently doing this to work around some networking issues from going outside the kubernetes cluster and then back in through ingress. seems like there would be a lot of good use cases for this sort of thing, and it'd be nice if the `coder_agent` resource had an attribute you could specify to override the domain the agent tries to phone home to.

something like...

```terraform
resource "coder_agent" "bird" {
coder_access_url = "coder.coder.svc.cluster.local" // idk, probably a bad name. workshop it first.
}
```