Fixes SDK-3084
## Changes:
- Added new `ENABLE_CUSTOM_ENTITLEMENT_COMPUTATION` compile time flag
- Hidden `configureInCustomEntitlementsModeWithApiKey` behind
`ENABLE_CUSTOM_ENTITLEMENT_COMPUTATION`
- Added new `RevenueCat_CustomEntitlementComputation` SPM target and
library
- Added CircleCI job for `swift build --target
RevenueCat_CustomEntitlementComputation`
- Added new `SPMCustomEntitlementComputationInstallation` using the new
`configureInCustomEntitlementsComputationMode`
- Added CircleCI job for `SPMCustomEntitlementComputationInstallation`
- Added `FrameworkDisambiguation.swift` to avoid using `RevenueCat.`
disambiguation for files that
- Changed API testers to only use the new methods if
`ENABLE_CUSTOM_ENTITLEMENT_COMPUTATION` is defined
- Updated `DangerousSettings` to not expose
`customEntitlementComputation` publicly. The only way to use that now
with `public` API is through
`Purchases.configureInCustomEntitlementsModeWithApiKey`
- Updated sample app to use new library
