### Motivation
Allow backend to determine Storefront of customer
### Description
- Adds `X-Storefront` as a default header
- Uses `StoreKit 1` implementation since its not `async`
- `StoreKit 2` implementation is `async` and would require a some
restructuring that we don't need right now
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Co-authored-by: NachoSoto <ignaciosoto90@gmail.com>
Similarly to #2262, test execution ordering was leading to [flaky code
coverage
results](https://app.codecov.io/gh/RevenueCat/purchases-ios/pull/2262)
with this.
Because we didn't have any test coverage, only if some other test
changed the `Storefront`, this test was calling the delegate code.
This adds explicit test coverage using a `MockAsyncSequence`.
### Other changes:
- Added `StorefrontType` to `StoreKit2StorefrontListenerDelegate` method
- `Storefront` initializers are no longer `Optional` `init?`
- Simplified `StoreKit2StorefrontListenerTests` by moving
`AvailabilityChecks` to `setUp`
Fixes https://community.revenuecat.com/sdks-51/objective-c-how-to-access-rcstorefront-1647?postid=5154#post5154
`RCStoreFront` does have a `currentStorefront` property which supports StoreKit 1 and StoreKit 2, but due to the fact that the SK2 way of obtaining the current storefront is `async`, this property is only `async`, which means that it indeed isn’t available on Objective-C.
This adds a way to be able to access the current storefront from Objective-C.
