agent config for plugins and templates doesn't filter invalid keys

[tgross]

Agent configurations are accepted as valid even if they have invalid keys in the plugin or client.template blocks. This can create confusion for operators (ex, using a config that was added in a later version of Nomad #8960 (comment)) and developers (ex. silently accepting invalid struct tags #8988 (comment))

An example configuration that should be treated as invalid (or at the very least warned against in the logs):

name = "test"
log_level = "DEBUG"
data_dir = "/var/nomad"

server {
  enabled          = true
  bootstrap_expect = 1
}

client {
  enabled = true

  template {
    whatevs = true
    function_blacklist = ["env"]
  }
}

plugin "qemu" {
  whatever = false
}

plugin "raw_exec" {
  whatever = false
  config {
    enabled = true
  }
}

plugin "docker" {
  whatever = false

  config {
    allow_privileged = true
  }
}