Validate examples

[lquerel]

This PR introduces validation for the examples attribute field. This validation ensures consistency between the attribute type and the type of the examples. For backward compatibility, an attribute type string[] will be compatible with examples containing a list of strings. In future mode, the only valid examples are a list of lists of strings to match the attribute type.

In future mode, the following attribute will not be valid:

id: attr
type: string[]
brief: bla
examples: ["us-central1", "us-east-1"]

However, this declaration will be valid:

id: attr
type: string[]
brief: bla
examples: [["us-central1"], ["us-east-1"]]

Note: With future mode disabled, both examples are considered valid for backward compatibility.

Example of a registry check with the future mode enabled: weaver registry check --registry <registry-path> --future

Closes #295