v0.2.0

The /v1/systemone wire format is now served by open decision models as well
as TypeSafe. This release lets one application talk to several of them.

• Named endpoints: config :jev, endpoints: [laya: [base_url: ...]] and endpoint: :laya
  per call, from Jev.HTTP.post/3 or a Jev.Server reply. config :jev, endpoint: changes
  the default from :typesafe. Jev.HTTP.endpoint/1 returns the resolved settings.
• A named endpoint sends no Authorization header unless it has an api_key, and its
  usd_per_million_input defaults to zero. Transport settings are inherited from the
  top-level configuration.
• Jev.reply/3 and Jev.cost/2 take the price. When a server omits confidence, it is
  computed from the probabilities as TypeSafe defines it.
• Telemetry metadata and Jev.Error carry the endpoint name.
• Jev.Wire: the response body as JSONCodec structs. A 200 whose body does not fit the wire
  format comes back as {:error, %JSONCodec.Error{}} instead of a crash, integers are accepted
  where the API documents numbers, unknown fields are ignored, and
  Jev.Wire.Response.schema/0 is the JSON Schema of what the client accepts. Adds the
  json_codec dependency.
• Jev.Test: respond/2 answers a Req.Test stub from a reply-shaped map, reading the questions
  from the request; error/3 sends a failure; body/2 is the pure half; request/1 reads the
  request. :plug becomes an optional dependency for these.
• Jev.Backend: a behaviour with one callback, post/3. Jev.HTTP implements it and stays the
  default; backend: per request or config :jev, backend: picks another, for an in-process
  model or a fake. Jev.Telemetry.span/4 is the shared telemetry every backend wraps its work in,
  so events carry backend and are identical whatever answered.
• Jev.confidence/2 is public: TypeSafe's confidence for a distribution.
• [:jev, :answer] fires only for questions the server answered. A partial reply used to crash the
  request.