[agents] New agent 'dispatch' for message routing

[eolivelli]

Fixes #454

Summary:

• this patch adds a new processor 'dispatch' that is able to dispatch the record to different topics depending on a condition

This is an example:

topics:
  - name: "input-topic"
    creation-mode: create-if-not-exists
  - name: "topic1"
    creation-mode: create-if-not-exists
  - name: "topic2"
    creation-mode: create-if-not-exists
  - name: "default-topic"
    creation-mode: create-if-not-exists
pipeline:
  - name: "Dispatch"
    type: "dispatch"
    input: input-topic
    output: default-topic
    id: step1
    configuration:
      routes:
         - when: properties.language == "en"
           destination: topic1
         - when: properties.language == "fr"
           destination: topic2
           action: dispatch
         - when: properties.language == "none"
           action: drop

Important notes:

• the routes are evaluated in order, as soon as one condition is verified the message is sent to the destination and the processing stops
• if no route matches then the record is sent to the "output", that can be a topic or another agent, as usual
• for each route you can define an action: dispatch or drop, the default is "dispatch" that means to send the message, with "drop" the message is silently discarded (not emitted to the downstream agent)

[nicoloboschi]

LGTM, just not very sure about the empty destination

[eolivelli]

@nicoloboschi I added support for "action: dispatch|drop" PTAL

[nicoloboschi]

please update here as well

[eolivelli]

sure

[nicoloboschi]

Can you fix it ?

[eolivelli]

sure

[nicoloboschi]

can we check for blank text and raise error if action == dispatch

[eolivelli]

sure, also on the planner side