feat: Add unix sockets

- Unix sockets run parallel to Named Pipes
  - Named Pipes will only run on non-Wine
  - If the game crashes, the next run will clean up an orphaned socket.
- Restructure RPC to be a bit tidier

Dalamud/Networking/Rpc/Service/ClientHelloService.cs

@@ -0,0 +1,120 @@
+using System.Threading.Tasks;
+
+using Dalamud.Data;
+using Dalamud.Game;
+using Dalamud.Game.ClientState;
+using Dalamud.Utility;
+
+using Lumina.Excel.Sheets;
+
+namespace Dalamud.Networking.Rpc.Service;
+
+/// <summary>
+/// A minimal service to respond with information about this client.
+/// </summary>
+[ServiceManager.EarlyLoadedService]
+internal sealed class ClientHelloService : IInternalDisposableService
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ClientHelloService"/> class.
+    /// </summary>
+    /// <param name="rpcHostService">Injected host service.</param>
+    [ServiceManager.ServiceConstructor]
+    public ClientHelloService(RpcHostService rpcHostService)
+    {
+        rpcHostService.AddMethod("hello", this.HandleHello);
+    }
+
+    /// <summary>
+    /// Handle a hello request.
+    /// </summary>
+    /// <param name="request">.</param>
+    /// <returns>Respond with information.</returns>
+    public async Task<ClientHelloResponse> HandleHello(ClientHelloRequest request)
+    {
+        var dalamud = await Service<Dalamud>.GetAsync();
+
+        return new ClientHelloResponse
+        {
+            ApiVersion = "1.0",
+            DalamudVersion = Util.GetScmVersion(),
+            GameVersion = dalamud.StartInfo.GameVersion?.ToString() ?? "Unknown",
+            ClientIdentifier = await this.GetClientIdentifier(),
+        };
+    }
+
+    /// <inheritdoc/>
+    public void DisposeService()
+    {
+    }
+
+    private async Task<string> GetClientIdentifier()
+    {
+        var framework = await Service<Framework>.GetAsync();
+        var clientState = await Service<ClientState>.GetAsync();
+        var dataManager = await Service<DataManager>.GetAsync();
+
+        var clientIdentifier = $"FFXIV Process ${Environment.ProcessId}";
+
+        await framework.RunOnFrameworkThread(() =>
+        {
+            if (clientState.IsLoggedIn)
+            {
+                var player = clientState.LocalPlayer;
+                if (player != null)
+                {
+                    var world = dataManager.GetExcelSheet<World>().GetRow(player.HomeWorld.RowId);
+                    clientIdentifier = $"Logged in as {player.Name.TextValue} @ {world.Name.ExtractText()}";
+                }
+            }
+            else
+            {
+                clientIdentifier = "On login screen";
+            }
+        });
+
+        return clientIdentifier;
+    }
+}
+
+/// <summary>
+/// A request from a client to say hello.
+/// </summary>
+internal record ClientHelloRequest
+{
+    /// <summary>
+    /// Gets the API version this client is expecting.
+    /// </summary>
+    public string ApiVersion { get; init; } = string.Empty;
+
+    /// <summary>
+    /// Gets the user agent of the client.
+    /// </summary>
+    public string UserAgent { get; init; } = string.Empty;
+}
+
+/// <summary>
+/// A response from Dalamud to a hello request.
+/// </summary>
+internal record ClientHelloResponse
+{
+    /// <summary>
+    /// Gets the API version this server has offered.
+    /// </summary>
+    public string? ApiVersion { get; init; }
+
+    /// <summary>
+    /// Gets the current Dalamud version.
+    /// </summary>
+    public string? DalamudVersion { get; init; }
+
+    /// <summary>
+    /// Gets the current game version.
+    /// </summary>
+    public string? GameVersion { get; init; }
+
+    /// <summary>
+    /// Gets an identifier for this client.
+    /// </summary>
+    public string? ClientIdentifier { get; init; }
+}