Add support to the request tagging

[sameh-farouk]

Is your feature request related to a problem? Please describe

Request tagging lets you attach small, well-known hints to each envelope so relays/services can alter behavior without changing the payload schema.

Describe the solution you'd like

The client should support tagging a request in a backward-compatible way. The relay/service should know how to interpret your custom tags.

Behavior today
Today, fail-fast is the main and only built-in tag supported in the relay:

• fail-fast: If TagFailFast is present, the relay should return an error immediately when the destination twin is offline, instead of queueing until expiry.