Extensibility of GraphQl Pickup Locations

Nobody has claimed this yet.

### Description (*)

From a frontend UI perspective it would be great if third party carrier pickup locations are available through the pickupLocations query endpoint and use the same mutations to select the shipping address.

### Expected behavior (*)

The `PickupLocations.items` type should be of value `[PickupLocationInterface]`

https://github.com/magento/inventory/blob/1.2-develop/InventoryInStorePickupGraphQl/etc/schema.graphqls#L12
```graphql
interface PickupLocationInterface {
pickup_location_code: String
}
type PickupLocation implements PickupLocation {
...
}
```

It might be better to move the PickupLocations graphql schema to its own separate module, since it will not strictly be an 'in store' pickup location?

### Benefits
I think this improves Magento's extensibility and offering a UI for carries to add their pickup locations gives more control to the developer.

### Additional information

I was thinking, shoudn't almost _all_ return types be interfaces, so extension developers can more easily extend endpoints in general?

### Alternatives

It isn't directly clear if this document applies: https://github.com/magento/architecture/blob/master/design-documents/graph-ql/best-practice/extensibility.md

It might be possible for PickupLocation implementers to add an additional field to a PickupLocation, but would be better if we're not jamming everything in a single type:

```graphql
type PostNlOpeningHour {
date: String!
from: String
to: String
}

type PickupLocation {
post_opening_hours: [PostNlOpeningHour]
}
```

Start with InventoryInStorePickupGraphQl/etc/schema.graphqls, especially the PickupLocations.items type and PickupLocationInterface definition. Read the referenced GraphQL extensibility best-practice document and inspect the pickupLocations query and shipping-address mutations. Done means third-party pickup locations can use the query and existing mutations without being forced into one concrete type.

Written by the indexing model from the issue text.