feat: missing values as objects (DPv2)

[pierrecamilleri]

Allow for DPv2 missing values as objects with labels.

Features :

Global architecture approach

• Class deserializing support the superset of v1 and v2
• Class properties are as most as possible only v2 properties, not storing the initial version if read from a descriptor. Round-trip of deserializing and reserializing will therefore return a canonical descriptor, possibly different from original input. This canonical form is however, idempotent (deserializing and reserializing gives the same descriptor).
• Class validation however needs to know the version (possibly inherited) to validate accordingly

Descriptor validation

• implementation handles v1 and v2. Validation triggers an error if using objects with explicit v1. Lenient validation : if no $schema is provided, no error is reported (despite the DP reference stating that default value is v1). Custom schemas/profiles are considered v2 by default.
• Unicity of values and labels validated
  • Unicity of "strings" in case of a list of strings is not validated, as this would be a non-essential breaking change

Sidenote

Reference does not seem to explicitely forbid that Package and Schema have contradicting $schema properties. However, this should not be a feature, so current implementation gives precedence on the parent's version.

TODO

• Version inheritence in metadata_validate