Metadata: create/update/get a variable description

[kyao]

The metadata API is defined here: https://datamart-upload.readthedocs.io/en/latest/api/
Implement that POST /datasets/id/variables method.
The minimal JOSN content is the name of the variable:

{
  "name": "Access to clean fuels and technologies for cooking"
}

If the optional property VariableID is given then use the value of the VariableID as the variable identifier within the dataset. Otherwise, automatically generate a variable identifier.

The method PUT /datasets/id/variables/id2 updates an existing variable given JSON description.
The method GET /datasets/id/variables/id2 return the variable JSON description.