feat(providers): NatsConnectionProvider (NATS.Net)

src/VolcanicCore.Providers/NatsConnectionProvider.cs

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+using System.Text.Json;
+using NATS.Client.Core;
+using VolcanicCore.Core;
+
+namespace VolcanicCore.Providers;
+
+public sealed class NatsConnectionProvider : IConnectionProvider
+{
+    public string Id => "nats";
+    public string DisplayName => "NATS";
+
+    public JsonSchema ConfigSchema { get; } = new(JsonDocument.Parse("""
+        {
+          "type": "object",
+          "required": ["url"],
+          "properties": {
+            "url":  { "type": "string", "title": "Server URL", "default": "nats://127.0.0.1:4222" },
+            "user": { "type": "string", "title": "User" },
+            "pass": { "type": "string", "title": "Password", "format": "password" }
+          },
+          "secretFields": ["pass"]
+        }
+        """).RootElement);
+
+    public IReadOnlySet<string> SecretFields { get; } =
+        new HashSet<string>(StringComparer.Ordinal) { "pass" };
+
+    public async Task<object> OpenAsync(JsonElement config, CancellationToken ct)
+    {
+        var url = config.GetProperty("url").GetString()!;
+        var opts = new NatsOpts { Url = url };
+        if (config.TryGetProperty("user", out var u) && u.ValueKind == JsonValueKind.String
+            && config.TryGetProperty("pass", out var p) && p.ValueKind == JsonValueKind.String)
+        {
+            opts = opts with { AuthOpts = new NatsAuthOpts { Username = u.GetString(), Password = p.GetString() } };
+        }
+        var conn = new NatsConnection(opts);
+        await conn.ConnectAsync();
+        return conn;
+    }
+
+    public async Task CloseAsync(object handle)
+    {
+        if (handle is NatsConnection conn) await conn.DisposeAsync();
+    }
+}

src/VolcanicCore.Providers/ServiceCollectionExtensions.cs

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+using Microsoft.Extensions.DependencyInjection;
+using VolcanicCore.Core;
+
+namespace VolcanicCore.Providers;
+
+public static class ServiceCollectionExtensions
+{
+    public static IServiceCollection AddVolcanicProviders(this IServiceCollection services)
+    {
+        services.AddSingleton<IConnectionProvider, NatsConnectionProvider>();
+        return services;
+    }
+}