Add minified json output format

[silvanocerza]

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• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

Adds a new output format.

• What is the current behavior?

The arduino-cli supports two output formats as of now: text (default) and json.
json is always pretty printed, this might make it difficult for some to parse it correctly, it also make the output much more longer than necessary with lots of empty space.

• What is the new behavior?

The arduino-cli now supports three output formats as of now: text (default), json and jsonmini.

Example output:

$ arduino-cli board list
Port         Protocol Type              Board Name                FQBN             Core
/dev/ttyACM0 serial   Unknown
/dev/ttyACM1 serial   Serial Port (USB) Arduino Mega or Mega 2560 arduino:avr:mega arduino:avr


$ arduino-cli board list --format json
[
  {
    "port": {
      "address": "/dev/ttyACM0",
      "label": "/dev/ttyACM0",
      "protocol": "serial",
      "protocol_label": "Serial Port (USB)",
      "properties": {
        "pid": "0x5385",
        "serialNumber": "HTK32",
        "vid": "0x27c6"
      }
    }
  },
  {
    "matching_boards": [
      {
        "name": "Arduino Mega or Mega 2560",
        "fqbn": "arduino:avr:mega"
      }
    ],
    "port": {
      "address": "/dev/ttyACM1",
      "label": "/dev/ttyACM1",
      "protocol": "serial",
      "protocol_label": "Serial Port (USB)",
      "properties": {
        "pid": "0x0242",
        "serialNumber": "85438303133351C080B1",
        "vid": "0x2341"
      }
    }
  }
]

$ arduino-cli board list --format jsonmini
[{"port":{"address":"/dev/ttyACM0","label":"/dev/ttyACM0","protocol":"serial","protocol_label":"Serial Port (USB)","properties":{"pid":"0x5385","serialNumber":"HTK32","vid":"0x27c6"}}},{"matching_boards":[{"name":"Arduino Mega or Mega 2560","fqbn":"arduino:avr:mega"}],"port":{"address":"/dev/ttyACM1","label":"/dev/ttyACM1","protocol":"serial","protocol_label":"Serial Port (USB)","properties":{"pid":"0x0242","serialNumber":"85438303133351C080B1","vid":"0x2341"}}}]

• Does this PR introduce a breaking change, and is
  titled accordingly?

Nope.

• Other information:

Fixes #1584.

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