Add initial MQTT support

[sergiimk]

Description

Relates to: #368

This PR allows to define a dataset like this:

kind: DatasetSnapshot
version: 1
content:
  name: temp
  kind: Root
  metadata:
    - kind: SetPollingSource
      fetch:
        kind: mqtt
        host: test.mosquitto.org
        port: 1883
        topics:
          - path: dev.kamu.example.mqtt.temp
            qos: AtMostOnce
      read:
        kind: NdJson
        ...

In this case kamu-cli will act as an MQTT client, subscribing to some topic(s).

Upon kamu pull it will connect to the MQTT broker and either:

• get nothing and exit after some configurable timeout
• get "retained" MQTT event - e.g. last temperature measurement that broker stores and sends upon subscription
• get a history of events it missed if MQTT QoS level is == AtLeastOnce or ExactlyOnce which makes broker buffer events for some time when subscriber is offline (the behavior is highly broker-specific)

Technical debt:

• MQTT breaks polling/push source separation as it's neither - related to IaC configurability for flow schedules and variables kamu-node#57 we will likely replace these metadata events with standalone Source manifest
• MQTT client will auto-acknowledge the events, so if ingest fails later those events will be lost - I will convert to manual ACKs in later PR
• Server / always listening mode for MQTT will be in a separate PR
• Making kamu-node into an MQTT broker can be done later if needed - for now it will act as a client, so user has to have their own broker and it needs to be accessible to kamu

Checklist before requesting a review

• Unit and integration tests added
• Compatibility:
  • Network APIs: ✅
  • Workspace layout and metadata: ❗
    • Relies on extended ODF schemas: Introduce MQTT source open-data-fabric/open-data-fabric#88 but change is backwards-compatible
  • Configuration: ✅
  • Container images: ❗
    • Added test dependency for rumqttd docker image, but it's very tiny and shouldn't impact builds
• Documentation:
  • Changelog: ✅
  • Public documentation: ✅: Document MQTT kamu-docs#22
• Downstream effects:
  • kamu-node: ❌
    • More work is needed to add server mode

[s373r]

A classy first step into the bigger world of small devices!

Added minor comment & question

[sergiimk]

Documentation PR: kamu-data/kamu-docs#22