SDK: Refactor Binding configuration

[lenny-goodell]

Rename Binding to Trigger and move the configuration for the types under Trigger . TOML will look like this:

[Trigger]
Type="http"

or

[Trigger]
Type="edgex-messagebus"
SubscribeTopics="events, edgex/events"
PublishTopic="example"
  [Trigger.MessageBus]
  Type = "zero"
      [Trigger.MessageBus.SubscribeHost]
      Host = "localhost"
      Port = 5563
      Protocol = "tcp"
      [Trigger.MessageBus.PublishHost]
      Host = "*"
      Port = 5565
      Protocol = "tcp"

or

[Trigger]
Type="external-mqtt"
SubscribeTopics="something"
PublishTopic="example"
  [Trigger.ExternalMqtt]
  Url = "tcp://localhost:1883"
  ClientId = "ThisAppService"
  ConnectTimeout = "5s"
  AutoReconnect = true
  KeepAlive = 5
  QoS = 0
  Retain = false
  SkipCertVerify = false
  SecretPath = ""
  AuthMode = "none"