[CWS] Generate .length field for string fields

[lebauce]

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• [ ] If applicable, the config template has been updated.

[paulcacheux]

I'm not sure I understand why len(container.id) was not possible. This solution seems to add a lot of noise in the documentation and a lot of noise in the generation template

[paulcacheux]

Can you rebase, and restrict the length field on specific fields only