Fetch offline entitlement mappings through Remote Config

[rickvdl]

(Don't merge before backend is ready)

Android equivalent of iOS PR RevenueCat/purchases-ios#7265

Description

Implements fetching the ProductEntitlementMapping through the remote config infrastructure. This will only be done when remote config is enabled. And will still fall back to the existing implementation when remote config is disabled through the kill-switch.

There's still an open question around handling the fallback if the kill-switch is activated later here

[rickvdl]

This method will fetch the PEM from the legacy implementation only when the remote config one fails / is not available. However it could happen that it is fetched successfully, but remote config is disabled in a later stage.

If we want to tackle this as well I guess we'd have to always fetch it from the legacy system still?

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.47%. Comparing base (0289c99) to head (40b147f).

Files with missing lines	Patch %	Lines
.../offlineentitlements/OfflineEntitlementsManager.kt	87.75%	0 Missing and 6 partials ⚠️
...otlin/com/revenuecat/purchases/PurchasesFactory.kt	94.11%	0 Missing and 1 partial ⚠️
...rchases/common/remoteconfig/RemoteConfigManager.kt	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3817      +/-   ##
==========================================
+ Coverage   80.43%   80.47%   +0.03%     
==========================================
  Files         408      409       +1     
  Lines       16890    16953      +63     
  Branches     2467     2479      +12     
==========================================
+ Hits        13586    13643      +57     
  Misses       2342     2342              
- Partials      962      968       +6     

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[tonidero]

Still reviewing but looking great! Just left some initial comments

[tonidero]

Hmm TBH, I'm wondering if we need to be so careful specially for PEM... Note that PEM does not change per user + we would trigger a new fetch immediately after changing the user that would override it, so the window of error is quite narrow. I guess this makes it a bit safer, but just wondering if it's worth the extra complexity TBH...