feat: iap upgrade and downgrade

[FlorianKe]

In-App Purchase: Upgrade and Downgrade

If a merchant has already purchased an in-app feature, they can now upgrade to a higher plan (e.g., from Basic to Pro) or downgrade to a lower plan (e.g., from Pro to Basic).

As part of the implementation, a new admin component was introduced based on the updated design. Additionally, the purchase process was redefined, since we now require a cart to check whether the merchant already owns the feature. This information is retrieved via a new SBP endpoint.

The SBP endpoint returns information about the merchant's current in-app purchases. Based on that data, we can determine if the requested feature is already owned and whether an upgrade or downgrade is applicable.

Furthermore, the api.in-app-purchases.cart.order call has now optional supports for an additional body parameter:

For upgrades:

"subscriptionChange": {
  "type": "upgrade",
  "currentInAppFeatureIdentifier": "identifier"
}

For downgrades:

"subscriptionChange": {
  "type": "downgrade",
  "currentInAppFeatureIdentifier": "identifier"
}

If the subscriptionChange parameter is not set, the system will treat the request as a new purchase.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.45%. Comparing base (b38765a) to head (af91e38).

Files with missing lines	Patch %	Lines
src/Services/StoreClient.php	0.00%	12 Missing ⚠️
src/Controller/InAppPurchasesController.php	0.00%	2 Missing ⚠️
src/Services/InAppPurchasesService.php	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #110      +/-   ##
============================================
+ Coverage     68.89%   76.45%   +7.56%     
- Complexity       66       69       +3     
============================================
  Files            35        8      -27     
  Lines          1064      361     -703     
  Branches        126        0     -126     
============================================
- Hits            733      276     -457     
+ Misses          302       85     -217     
+ Partials         29        0      -29     

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[mstegmeyer]

I'm missing a description for this PR and this whole feature. This has already been very sidelined and not well documented, to have this at least here would be at least something 💙

[En0Ma1259]

Just minor things, except the language

stale