feat: do not return hidden variables

[dido18]

Motivation

Some brick variables must not be shown via the AppLab since they are "internal" variables of the bricks.

An example of such variables is CUSTOM_MODEL_PATH.

Change description

Support the hidden boolean variable added in the brick index to hide variables.

• do not return the variables in the GET /v1/bricks/{id} and GET /v1/apps/{id}/bricks/{id} with hidden:true
• A hidden variable could be added "manually" to the app.yaml

Additional Notes

Related pr - arduino/app-bricks-py#38

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.