fix(api): allow missing required variable when a brick is added + app validation

[dido18]

Motivation

Close #68

When a brick with required variables is added to an app created from scratch the operation fails.

This is because the FE (appLab) does not pass the required variables when the PUT /v1/apps/{appID}/bricks/{brickID} is called.

For example, the brick arduino:arduino_cloud has the ARDUINO_DEVICE_ID and ARDUINO_SECRET required.
When it is added to a scratch app, it fails silently.

Additionally, the issue does not occur when an app with a brick already added is cloned.
This is because when an app is cloned the code does not check the required variable.

Change description

The following changes are

• When a brick is added to an app: DO NOT raise an error by logging a Warning and putting the required variable as an empty string.
• When an app is started or restarted, a validation of the bricks of the app.yaml is performed. An app.yaml is invalid in the following cases:
  1. A required variable is omitted
  2. A required variable among two is omitted
  3. A variable does not exist into the brick definition
  4. A brickID not found in the bricks index

Example:
Given the following app-yaml, where the required ARDUINO_SECRET variable of arduino:arduino_cloud is missing:

name: hello
description: ""
ports: []
bricks:
- arduino:arduino_cloud: 
  variables:
      ARDUINO_DEVICE_ID: ""
icon: 😀

When an app is started

• from arduino-app-cli

$ arduino-app-cli app start user:ciao
[ERROR] Variable "Arduino_secret" Is Required By Brick "Arduino:arduino_cloud"

• from the FE:

variable "ARDUINO_SECRET" is required by brick "arduino:arduino_cloud" variable "ARDUINO_DEVICE_ID" is required by brick "arduino:arduino_cloud"

When an app is restarted

arduino@merola:/$ arduino-app-cli app restart user:ciao
[ERROR] Variable "Arduino_device_id" Is Required By Brick "Arduino:arduino_cloud"
Variable "Arduino_secret" Is Required By Brick "Arduino:arduino_cloud"
arduino@merola:/$

Additional Notes

Reviewer checklist

• PR addresses a single concern.
• PR title and description are properly filled.
• Changes will be merged in main.
• Changes are covered by tests.
• Logging is meaningful in case of troubleshooting.