[bricks] model configuration - refactoring for list of variables

[rjtokenring]

Allow to specify a per brick configuration in model's list file.
Migrate compose files variable definition inside bricks config to allow to specify more metadata

[dido18]

I would split the PR.

• PR#1. [NO-ISSUE] Remove the model_configuration_variables from the brick-config.yaml. The arduino-app-cli does not read/use it.
• PR#2. [BREAKING] Move the model_configuration under the model-list.yaml and put the configuration variable for each brick. This is a breaking change.
• PR#X [Enhancement] When needed, open PR to add the boolean field into the brick_config.yaml. Each addition should be discussed and clearly described

private: true  => the variable is only used internally (i.e., the model path)
mandatory: true => if the variable must be filled 
secret: true           => a variable that is a secret. 

[rjtokenring]

I would split the PR.

• PR#1. [NO-ISSUE] Remove the model_configuration_variables from the brick-config.yaml. The arduino-app-cli does not read/use it.
• PR#2. [BREAKING] Move the model_configuration under the model-list.yaml and put the configuration variable for each brick. This is a breaking change.
• PR#X [Enhancement] When needed, open PR to add the boolean field into the brick_config.yaml. Each addition should be discussed and clearly described

private: true  => the variable is only used internally (i.e., the model path)
mandatory: true => if the variable must be filled 
secret: true           => a variable that is a secret. 

I do not agree. model_configuration_variables can be removed when the logic informatio is migrated into the new data model.
Some for the last point. Here we are changing the variable declaration data model and I think it is better to define it in a single PR.