feat: add registerPlugin so a plugin can be added after the client starts

[abelonogov-ld]

Summary

Plugins could only be supplied through LDConfig, so an integration that learns about a plugin later — or that wants to instrument a client it did not configure — had no way in. This adds LDClient.registerPlugin(_:), matching the method the .NET and Flutter SDKs already expose and the one being added to the Android SDK in launchdarkly/android-client-sdk#393.

• Hooks became mutable, because registration after start cannot extend a constant array. They now live in a private var behind an NSLock and are replaced rather than mutated in place. Each series reads a snapshot once into a local, so the hooks a series ends with are the hooks it began with — were the array read again mid-series, a hook registered in between would be handed an after stage for a series whose before stage it was never in. The three read sites (evaluateWithHooks, executeBeforeIdentifyHooks, executeAfterTrackHooks) all take that snapshot up front.
• Hooks go live only once register returns. This matches the Android and .NET ordering, and differs from configuration-time registration where a plugin's hooks are active before register is called. So a plugin's own hooks will not observe evaluations or identify calls its register makes; everything afterwards does run them.
• EnvironmentMetadata is retained on the instance, so a plugin registered later is handed the same environment description as one configured up front. This also removes the duplicate construction that previously existed in both start and collectHooks.

Registration applies to the one client it is called on, so a multi-environment setup means calling it per environment.

Note that the iOS Plugin protocol has no onPluginsReady, so unlike Android this path is just getHooks then register then activate.

Test plan

Five cases added to LDClientPluginsSpec, all passing on an iPhone 16e simulator alongside the two pre-existing plugin tests (7 total):

• testRegisterPluginPassesClientAndEnvironmentMetadata
• testRegisterPluginActivatesBundledHooks
• testRegisterPluginDoesNotRunTheRegisteringPluginsOwnHooks
• testRegisterPluginHooksRunAfterConfiguredHooks
• testRegisterPluginAppliesOnlyToTheClientItIsCalledOn
• SwiftLint introduces no new violations in the touched files

---

Note

Overview
Adds LDClient.registerPlugin(_:) so a plugin can be attached after start, not only via LDConfig.plugins. Registration is per client (call it on each environment in a multi-env setup).

Hooks are no longer a constant array: they sit behind an NSLock, and each evaluation/identify/track series snapshots them once so a hook registered mid-series never gets an after without a matching before. register runs before those hooks go live, so a plugin’s own hooks do not observe work done inside register. EnvironmentMetadata is stored on the client so late plugins get the same description as config-time ones.

^{Reviewed by Cursor Bugbot for commit 5798e94. Bugbot is set up for automated code reviews on this repo. Configure here.}