Add a smooth transition to the anomaly API

[SebastianSchmidl]

Some existing anomaly kinds use smooth transitions. We should allow all (except extremum) anomalies to do this, and therefore move it to the anomaly itself instead of the anomaly kind.

Example configuration:

timeseries:
  - name: "test"
    base-oscillations:
      - kind: "sine"
    anomalies:
      - position: beginning
        length: 100
        transition-window: 10  # <--
        kinds:
          - kind: "amplitude"
            amplitude_factor: 2