[Marketplace] Adds the ability to set metafields on cart items

[mikemurray]

This PR adds the ability to add metafields to a cart item while adding that item to cart. This could be useful to apply additional information to an item added like affiliate tracking or geolocation, etc.

Examples:

Adds metadata to a cart item only once, and does not get updated if the same item is added to the cart again.

const additionalOptions = {
  metafields: [
    { key: "KEY", value: "VALUE" }
  ]
};

Meteor.call("cart/addToCart", "PRODUCT_ID", "VARIANT_ID", QUANTITY, additionalOptions);

Adds metadata to a cart item while adding to cart, and will update every time that item is added to cart.

const additionalOptions = {
  overriteExistingMetafields: true,
  metafields: [
    { key: "KEY", value: "VALUE" }
  ]
};

Meteor.call("cart/addToCart", "PRODUCT_ID", "VARIANT_ID", QUANTITY, additionalOptions);

Final param, additionalOptions is optional.

Meteor.call("cart/addToCart", "PRODUCT_ID", "VARIANT_ID", QUANTITY);

[brent-hoover]

typo?

[mikemurray]

good catch

[brent-hoover]

Typo in option name?

[brent-hoover]

My only concern here is that Metadata not turn into ActualData. We need to not store anything there that the app relies on to do any primary logic, otherwise it should be in the schema.

[spencern]

Looks good to me.