Add CarAssets zod schema

[dan-gamble]

When running this:

    const data = await step.do("get car asset data", async () => {
      const tokenResult = await getAdminAccessToken();
      if (tokenResult.error != null) {
        throw new NonRetryableError(tokenResult.error)
      }

      try {
        const client = createIracingApiClient(tokenResult.accessToken, tokenResult.refreshToken, tokenResult.expiresAt)

        return await client.car.assets()
      } catch (error: any) {
        if (error instanceof IRacingError) {
          console.error("iRacing API Error", error.message);
          console.error("Status:", error.status);

          throw error;
        } else {
          console.error("Unexpected error", error);

          throw error;
        }
      }
    });

The type I'm getting is () => Promise<Record<string, unknown>>

If I then go and look at the iracing-data-client/lib/index.d.ts I'm seeing the below for CarAssets

declare const CarAssets: z.ZodMiniRecord<z.ZodMiniString<string>, z.ZodMiniUnknown>;

As opposed to something like this for series:

declare const SeriesAssets: z.ZodMiniRecord<z.ZodMiniString<string>, z.ZodMiniObject<{
    largeImage: z.ZodMiniOptional<z.ZodMiniNullable<z.ZodMiniString<string>>>;
    logo: z.ZodMiniOptional<z.ZodMiniNullable<z.ZodMiniString<string>>>;
    seriesCopy: z.ZodMiniOptional<z.ZodMiniNullable<z.ZodMiniString<string>>>;
    seriesId: z.ZodMiniOptional<z.ZodMiniNullable<z.ZodMiniNumber<number>>>;
    smallImage: z.ZodMiniOptional<z.ZodMiniNullable<z.ZodMiniString<string>>>;
}, z.core.$strip>>;

The documentation has it

I need to double check the types as I've just made them all nullable