Validation on measurement transformation language

[tstorek]

Is your feature request related to a problem? Please describe.
Currently, the measurement transformation language provided in the iot agents is not validated.

Describe the solution you'd like
When a device configuration is invoked or changed by a user the device model should check if the expression language is used in a consistant way. We can use pydantic validators for this.
As FIWARE recommends using JEXL I suggest trying the pyjexl package for validation.

Additional context

• Add any other context or screenshots about the feature request here.
• https://pypi.org/project/pyjexl/

[github-actions]

Branch 165-Validation-on-measurement-transformation-language created!

[djs0109]

merged in #260