[ts] Add private_metafield_namespaces property to webhook types

[kristianpd]

When registering webhooks with shopify you can specify which metafield_namespaces you want them to include when they push webhooks.

You can also register for a separate set of private_metafield_namespaces. Private metafields are only available to the app that creates them vs regular metafields which are available to everyone.

This updates the types of webhooks to include this property.

[lpinca]

Can you please revert the fixtures changes and keep only the TypeScript type definitions changes?

[coveralls]

Coverage remained the same at 100.0% when pulling 06aca90d0321c4fe296e310cb8e041022506ce9f on kristianpd:add-private-metafield-namespaces into 76779d6e1b8c32f0ca794dcfb613b289b7e8524c on MONEI:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 06aca90d0321c4fe296e310cb8e041022506ce9f on kristianpd:add-private-metafield-namespaces into 76779d6e1b8c32f0ca794dcfb613b289b7e8524c on MONEI:master.

[lpinca]

Thank you.