deny_unauthorized is not allowed as a proxy protocol behavior

[bmendric]

Describe the bug
Setting proxy_protocol_behavior = "deny_unauthorized" fails config validation

To Reproduce
Steps to reproduce the behavior:

1. Create a Vault server config with the above setting
2. Attempt to run vault server -config=config.hcl

Expected behavior
I do not expect an error on valid configurations

Environment:

• Vault Server Version (retrieve with vault status): N/a
• Vault CLI Version (retrieve with vault version): 1.16.1
• Server Operating System/Architecture:

Vault server configuration file(s):

storage "inmem" {}

listener "tcp" {
  address = "[::]:8200"
  tls_disable = true
  proxy_protocol_behavior = "deny_unauthorized"
  proxy_protocol_authorized_addrs = "127.0.0.1"
}

log_level = "trace"
log_format = "json"
ui = false

disable_mlock = true

Additional context
I was working to validate that #17144 is still a problem. I originally tested this configuration with 1.15.5 (still exhibits the above problem, but does accept this config.hcl), however, I upgraded to 1.16.1 for sanity and discovered this. So I assume its something that broke in 1.16.0.

Also, here is the supporting documentation saying this is valid: https://developer.hashicorp.com/vault/docs/configuration/listener/tcp#proxy_protocol_behavior