Add GenericType to GatsbyFunctionRequest

[njbmartin]

When using typescript, it would be great if GatsbyFunctionRequest supported GenericTypes, so that we don't have to cast the type later.

/**
 * Gatsby function route request
 */
export interface GatsbyFunctionRequest<T = any> extends IncomingMessage {
  /**
   * Object of values from URL query parameters (after the ? in the URL)
   */
  query: Record<string, string>

  /**
   * Object of values from route parameters
   */
  params: Record<string, string>
  body: T
  /**
   * Object of `cookies` from header
   */
  cookies: Record<string, string>
}

[LekoArts]

Done in #35029