Fixes #23332: Serde errors do not point to the correct line number

[amousset]

https://issues.rudder.io/issues/23332

Solves probably 75% of the problem with a low effort.

The idea is to avoid parsing an untagged enum with serde (as it totally breaks error reporting), and use a plain struct instead, and then convert manually to the different kinds.
The higher level errors still do not point to a line in the source (but it was never the case, and would require heavy lifting, like replacing serde with a lower level YAML parser), but all reporting error line are now correct.

[amousset]

PR updated with a new commit

[Normation-Quality-Assistant]

This PR is not mergeable to upper versions.
Since it is "Ready for merge" you must merge it by yourself using the following command:
rudder-dev merge https://github.com/Normation/rudder/pull/5226
-- Your faithful QA
Kant merge: "All our knowledge begins with the senses, proceeds then to the understanding, and ends with reason. There is nothing higher than reason."
(https://ci.normation.com/jenkins/job/merge-accepted-pr/76935/console)

[amousset]

OK, squash merging this PR