Document attribute length constraint

[SijmenHuizenga]

https://github.com/eclipse/hawkbit/blob/541a0129ef77281b8df5cad0a5de38cca359df1b/hawkbit-repository/hawkbit-repository-api/src/main/java/org/eclipse/hawkbit/repository/model/Target.java#L38-L46

This shows length constraints on controller attribute key and value. I couldn't find any reference to this in the documentation. I propose we add these maximum values to the api docs.

[AzazelSensei]

Opened #3341 with the 128-character key and value limits on the DDI API page.