Do not merge before v2 release feat: register plugin dynamically

[abelonogov-ld]

Summary

Adds runtime plugin registration through LDClient.registerPlugin. This API is not supposed to be used by Customers.

• Stores plugin environment metadata during client construction so runtime plugins receive the same SDK, application, and credential metadata as config-time plugins.
• Registers plugin-provided hooks before invoking plugin.register, so hooks are active during registration.
• Skips plugin registration if reading plugin.hooks fails, while preserving existing safe handling for registration exceptions.

Test Plan

• Added tests for runtime plugin registration metadata.
• Added tests verifying runtime plugin hooks are active after registration.
• Added coverage that hooks are installed before plugin.register runs.
• Added failure coverage for throwing hooks getters and throwing register methods.
• Ran:

flutter test test/ld_client_plugin_test.dart


<!-- CURSOR_SUMMARY -->
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> [!NOTE]
> **Medium Risk**
> Touches SDK plugin/hook registration and evaluation order during `register`, which can affect observability plugins; changes are covered by tests and failures are contained via existing safe-registration helpers.
> 
> **Overview**
> Adds **`LDClient.registerPlugin`** so plugins can be attached after the client is built (intended for internal use, not customer-facing). The client now keeps **`PluginEnvironmentMetadata`** from construction and reuses it for runtime plugins so they get the same SDK, app, and credential metadata as config-time plugins.
> 
> **`registerPlugin`** installs bundled hooks via **`addHook`** before calling **`Plugin.register`**, skips registration if **`plugin.hooks`** throws, and logs (without rethrowing) if **`register`** throws. Tests cover metadata, hook activation, hook-before-register ordering, and both failure paths.
> 
> Also bumps **`launchdarkly_flutter_client_sdk`** to **4.16.0** / **`launchdarkly_common_client`** **1.10.0**, refreshes contract-test **`pubspec.lock`** files, and ignores regenerated CocoaPods **`Podfile`** / **`Podfile.lock`** in the Flutter example **ios** and **macos** trees.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit c815251ba62a9bba334e13d4481ef704218dbaca. Bugbot is set up for automated code reviews on this repo. Configure [here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

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