added thing message

[alpsarica]

Too solve the issues in Agent package (adding support for twin features and messages) thing message definition is added.

[nacidai]

Two changes are needed:

1. Please target the new enhancement branch not the main.
   2). The headers field in ditto messages includes some important data, see ProtocolEnvelope. So please make the headers field explicitly modeled to include the fields from the following example.

"headers": {
"reply-to":"att/cbcbcbc",
"ditto-originator":"nginx:ditto",
"response-required":false,"
"content-type":"application/json",
"correlation-id":"xccv-bababab-09998989-2424242"
}

[alpsarica]

@nacidai made the requested changes. Added ThingHeaders message to the Thing message.

[alpsarica]

added correlation-id to ThingHeader message

[nacidai]

Merging changes related. to #3