refactor(core): move global environment behind KlaviyoEnv holder for Swift 6#568

ndurell wants to merge 2 commits intomasterklaviyo/klaviyo-swift-sdk:masterfrom

refactor/nd-20260429-klaviyo-env-holderklaviyo/klaviyo-swift-sdk:refactor/nd-20260429-klaviyo-env-holderCopy head branch name to clipboard