Add a schema facet to the dataset search

[AntoineAugusti]

This PR adds a new facet to the dataset search. This will be used to identify resources that satisfy a schema, declared through the schema attribute of resources.

This new facet enables us to identify datasets with resources satisfying a schema. The API endpoint /api/1/datasets/?schema=etalab/schema-irve will list datasets embedding at least a resource satisfying the etalab/schema-irve schema.

We will be able to monitor how many datasets contain resources with schema attribute set with the endpoint /api/1/datasets/?facets=schema

⚠️ This PR requires a reindexation of datasets. This can be done with udata search index datasets

[abulte]