MQTT: handle custom relay status payloads

[mcspr]

• customize TOGGLE payload
• match payload string when receiving data
• reference enum values instead of raw integers, spell out intended status
• remove dead code

amend #1885, capitalize relayPayload... suffix instead of using uppercase
add relayPayloadToggle

[mcspr]

cc @induprakash from #1085
General idea is configure both incoming and outgoing on/off/toggle payload strings from runtime. Outgoing is strictly from the setting, incoming is lax parsing matching old 0/1/2 first, then custom payloads and then "off"/"on"/"toggle" (case insensitive)