[OpenAPI] Add endpoint to get multiple service definitions at once

[slinkydeveloper]

Add an endpoint to get multiple openapi definitions at once for multiple services.

To handle ambiguity, users should add in the service metadata, at discovery, something like dev.restate.openapi.prefix metadata, that is used to add prefixes to schemas.

In case of ambiguity, the endpoint should fail to generate the OpenAPI, rather than silently overwriting stuff.