Omit readonly properties from PATCH requestContent

If a schema is referenced as the requestContent to an API, the generated RequestBody should omit readonly properties.

Given a user:

```ts
export interface User {
readonly id: number
name: string
readonly email: string
}
```

When this type is referenced to be the input to a PATCH request, the input argument should omit readonly properties.

It should be possible to do some Typescript magic to strip readonly properties, like mentioned in https://stackoverflow.com/a/49579497/2240706.

```ts
type IfEquals =
(() => T extends X ? 1 : 2) extends
(() => T extends Y ? 1 : 2) ? A : B;

type WritableKeys = {
[P in keyof T]-?: IfEquals<{ [Q in P]: T[P] }, { -readonly [Q in P]: T[P] }, P>
}[keyof T];

type ReadonlyKeys = {
[P in keyof T]-?: IfEquals<{ [Q in P]: T[P] }, { -readonly [Q in P]: T[P] }, never, P>
}[keyof T];

type OmitReadonly = Pick>;
```

It would then be a case of updating the generated code to declare the argument as `OmitReadonly`