feat(schema): Add schema resolver converter functionality

[daffl]

This pull request adds a converter option to schema resolvers which allow to convert existing data into a completely different format. Converters always run before validation and property resolvers.

    const userConverterResolver = resolve<User, typeof context>({
      schema: userSchema,
      validate: 'before',
      converter: async data => ({
        firstName: 'Default',
        lastName: 'Name',
        ...data
      }),
      properties: {
        name: async (_name, user) => `${user.firstName} ${user.lastName}`
      }
    });