feat: Add .NET Client SDK examples (console + MAUI)

[jsonbailey]

Summary

Migrates both examples from launchdarkly/hello-dotnet-client into examples-dotnet, following the EXAM layout.

Layout

client-sdk/
├── README.md                       # index (Getting Started + Frameworks)
├── getting-started/                # console app (Flag Retrieval)
│   ├── DotNetConsoleApp.csproj
│   ├── Program.cs
│   ├── README.md
│   └── NOTICE                      # Copyright 2023
└── frameworks/
    └── maui/                       # .NET MAUI app (Android & iOS)
        ├── MauiApp.csproj
        ├── global.json             # scoped here, not repo root
        ├── Platforms/ Resources/ ...
        ├── README.md
        └── NOTICE                  # Copyright 2023

• Partition named after the package (LaunchDarkly.ClientSdk → client-sdk/).
• Console → getting-started/; MAUI → frameworks/maui/ (per EXAM, frameworks hold runtime-specific examples).
• global.json (SDK 8.0) scoped to the MAUI dir so it doesn't constrain the other examples.
• MAUI appsettings.json (holds the mobile key) is gitignored; appsettings.example.json is committed.
• Per-example NOTICE files carry the old repo's 2023 copyright; all migrated text files BOM-stripped; binary assets (fonts/images) copied as-is.

CI (client.yml, ported from the old repo's recipe)

• build-and-run (ubuntu): build console, fetch mobile/flag keys, configure MAUI appsettings, install maui-android, build the MAUI APK, run it on an Android emulator (verify-maui-android.sh), and run the console app via verify-hello-app (mobile key).
• build-maui (macOS): build MAUI for Android + iOS.
• Actions pinned per convention (third-party → SHA + version; LD actions → major tag).

Notes / caveats

• The MAUI/emulator CI can't be verified locally (no MAUI workload/emulator here) — it'll get its first real run on this PR; may need an iteration. The console app builds clean locally.
• This relies on the OIDC trust + use_mobile_key SSM params (/sdk/common/hello-apps/mobile-key), already used by the old repo.
• Reconciliation with feat: Add .NET Server AI SDK examples #2: when build.yml (introduced in feat: Add .NET Server AI SDK examples #2) lands, its glob should exclude client-sdk/frameworks/maui/ (MAUI can't build in the generic loop); the console is fine to keep in the generic build.

Part of SDK-2559. Covers the migration portion of SDK-2561 (old-repo README repoint + Terraform archive tracked on that ticket).

Jira: SDK-2561

[jsonbailey]

Updated to per-slice CI: the workflow is now client-sdk.yml, path-filtered to client-sdk/**. This supersedes the description's "reconciliation with #2 / exclude MAUI from build.yml" note — there's no generic build.yml to reconcile with anymore.