fix(curve): remove if then else and replace with top level oneOf

[albieduffy]

No description provided.

[mattelen]

👀

[olliemath]

@albieduffy @mattelen an alternative here is just to have essentially two separate schemas for the properties instead of adding the oneOf in the individual fields and enforcing separate cross-field validation later on in the schema:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "Curve Schema",
  "description": "A Curve represents a series of points on a plot. Typically, interest rates, volatility or forward prices.",
  "type": "object",
  "oneOf": [
    {... numeric curve properties ...},
    {... text curve properties ...}
  ]
}

That seems clearer to me than a separate validation section (hopefully it would also allow consumers to model this as either a RiskRatingCurve or GenericCurve type) - any reason to prefer enforcing further down in the schema?