Add custom metadata in organization

[quaxsze]

fix datagouv/data.gouv.fr#1183

[quaxsze]

Behavior precision:

• The organization custom metadata dict can only have key in the following: ["title", "description", "type", "choices"].
• The type value can only be one of those: ["str", "int", "float", "bool", "datetime", "date", "choice"]
• The dataset custom metadata must have been defined in the dataset's organization and be of the right type
• If the organization changes the type of a custom metada, it is not retroactive. Validation error will only occur on future modification of the dataset.
• If the organization deletes a custom metada, it is not retroactive. Validation error will only occur on future modification of the dataset.

[maudetes]

I wonder if we could be a bit more concise?

[maudetes]

Thank you!