As a tool builder I want to read and edit Entity Schemas via an API in order to make use of them in my tools.

Problem:
Right now no proper API for Entity Schemas exists that would allow tool builders to interact with them. All that is available is Special:EntitySchemaText, which returns the Schema but for example not the label, description and aliases for the Schema.

We should have an API that lets tool builders read and write Entity Schemas.

BDD
GIVEN
AND
WHEN
AND
THEN
AND

Acceptance criteria:

• an API exists for reading and writing Entity Schemas. It includes reading and writing for all Entity Schema content (schema, labels, descriptions, aliases)

Open questions:

• This should probably be part of the Wikibase REST API or something close to it?