refactor: move validate command under tdp vars

[PaulFarault]

Which issue(s) this PR fixes

Fixes None

Additional comments

It makes more sense to move the variable validation command (currently tdp validate) in the tdp vars sub-commands groups (tdp vars validate as a result).

Agreements

• [X] I hereby declare this contribution to be licensed under the Apache License Version 2.0, January 2004.
• [X] I hereby agree to grant TOSIT a copyright license to use my contributions.