Release v0.4.0

Features

• Support Pydantic BaseModel automated conversion to
  StructType/StructValue.
• UnpackRow base dataclass for defining rows that should be unpacked in Tierkreis
  (e.g. multiple outputs in a worker function).

Breaking changes

• TierkreisStruct and register_struct_convertible are removed. Now automated
  conversion will be attempted on any dataclass or Pydantic BaseModel.
• ServerRuntime simplified to only offer run_graph for execution, all "task"
  related methods removed. runtime.proto updated to v1alpha1 to match.

Minor changes

• CircStruct simplified to just contain the JSON string of a Pytket Circuit
  (essentially acts as an opaque newtype).