feat: Initialize Zastava Webhook service with TLS and Authority authentication

- Added Program.cs to set up the web application with Serilog for logging, health check endpoints, and a placeholder admission endpoint.
- Configured Kestrel server to use TLS 1.3 and handle client certificates appropriately.
- Created StellaOps.Zastava.Webhook.csproj with necessary dependencies including Serilog and Polly.
- Documented tasks in TASKS.md for the Zastava Webhook project, outlining current work and exit criteria for each task.

src/StellaOps.Authority/StellaOps.Authority.Storage.Mongo/Sessions/AuthorityMongoSessionAccessor.cs

@@ -0,0 +1,128 @@
+using Microsoft.Extensions.Options;
+using MongoDB.Driver;
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using StellaOps.Authority.Storage.Mongo.Options;
+
+namespace StellaOps.Authority.Storage.Mongo.Sessions;
+
+public interface IAuthorityMongoSessionAccessor : IAsyncDisposable
+{
+    ValueTask<IClientSessionHandle> GetSessionAsync(CancellationToken cancellationToken = default);
+}
+
+internal sealed class AuthorityMongoSessionAccessor : IAuthorityMongoSessionAccessor
+{
+    private readonly IMongoClient client;
+    private readonly AuthorityMongoOptions options;
+    private readonly object gate = new();
+    private Task<IClientSessionHandle>? sessionTask;
+    private IClientSessionHandle? session;
+    private bool disposed;
+
+    public AuthorityMongoSessionAccessor(
+        IMongoClient client,
+        IOptions<AuthorityMongoOptions> options)
+    {
+        this.client = client ?? throw new ArgumentNullException(nameof(client));
+        this.options = options?.Value ?? throw new ArgumentNullException(nameof(options));
+    }
+
+    public async ValueTask<IClientSessionHandle> GetSessionAsync(CancellationToken cancellationToken = default)
+    {
+        ObjectDisposedException.ThrowIf(disposed, this);
+
+        var existing = Volatile.Read(ref session);
+        if (existing is not null)
+        {
+            return existing;
+        }
+
+        Task<IClientSessionHandle> startTask;
+
+        lock (gate)
+        {
+            if (session is { } cached)
+            {
+                return cached;
+            }
+
+            sessionTask ??= StartSessionInternalAsync(cancellationToken);
+            startTask = sessionTask;
+        }
+
+        try
+        {
+            var handle = await startTask.WaitAsync(cancellationToken).ConfigureAwait(false);
+
+            if (session is null)
+            {
+                lock (gate)
+                {
+                    if (session is null)
+                    {
+                        session = handle;
+                        sessionTask = Task.FromResult(handle);
+                    }
+                }
+            }
+
+            return handle;
+        }
+        catch
+        {
+            lock (gate)
+            {
+                if (ReferenceEquals(sessionTask, startTask))
+                {
+                    sessionTask = null;
+                }
+            }
+
+            throw;
+        }
+    }
+
+    private async Task<IClientSessionHandle> StartSessionInternalAsync(CancellationToken cancellationToken)
+    {
+        var sessionOptions = new ClientSessionOptions
+        {
+            CausalConsistency = true,
+            DefaultTransactionOptions = new TransactionOptions(
+                readPreference: ReadPreference.Primary,
+                readConcern: ReadConcern.Majority,
+                writeConcern: WriteConcern.WMajority.With(wTimeout: options.CommandTimeout))
+        };
+
+        var handle = await client.StartSessionAsync(sessionOptions, cancellationToken).ConfigureAwait(false);
+        return handle;
+    }
+
+    public ValueTask DisposeAsync()
+    {
+        if (disposed)
+        {
+            return ValueTask.CompletedTask;
+        }
+
+        disposed = true;
+
+        IClientSessionHandle? handle;
+
+        lock (gate)
+        {
+            handle = session;
+            session = null;
+            sessionTask = null;
+        }
+
+        if (handle is not null)
+        {
+            handle.Dispose();
+        }
+
+        GC.SuppressFinalize(this);
+        return ValueTask.CompletedTask;
+    }
+}