Introduce a schema mapping configuration

**Motivation**
To validate resources, clients need today to also provide the schema which is error prone and forces them to keep track of the schema (included the version/tag) to use.

Defining a schema mapping by type in Delta would make it easier:
* The user would only provide the resource type
* Delta would deduce the schema to use from its type

This first ticket deals with introducing a new global entity responsible for this mapping between a concrete type and its associated schema.

**Acceptance criteria**
1. An endpoint is introduced to update all the entries of this new configuration
2. An endpoint is provided to fetch this mapping (latest and specific revision)

**Not in scope**
1. Implementation in resources