Add Authority Advisory AI and API Lifecycle Configuration

- Introduced AuthorityAdvisoryAiOptions and related classes for managing advisory AI configurations, including remote inference options and tenant-specific settings.
- Added AuthorityApiLifecycleOptions to control API lifecycle settings, including legacy OAuth endpoint configurations.
- Implemented validation and normalization methods for both advisory AI and API lifecycle options to ensure proper configuration.
- Created AuthorityNotificationsOptions and its related classes for managing notification settings, including ack tokens, webhooks, and escalation options.
- Developed IssuerDirectoryClient and related models for interacting with the issuer directory service, including caching mechanisms and HTTP client configurations.
- Added support for dependency injection through ServiceCollectionExtensions for the Issuer Directory Client.
- Updated project file to include necessary package references for the new Issuer Directory Client library.

src/Attestor/StellaOps.Attestor.Envelope/DsseEnvelope.cs

@@ -0,0 +1,48 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace StellaOps.Attestor.Envelope;
+
+public sealed class DsseEnvelope
+{
+    public DsseEnvelope(
+        string payloadType,
+        ReadOnlyMemory<byte> payload,
+        IEnumerable<DsseSignature> signatures,
+        string? payloadContentType = null,
+        DsseDetachedPayloadReference? detachedPayload = null)
+    {
+        if (string.IsNullOrWhiteSpace(payloadType))
+        {
+            throw new ArgumentException("payloadType must be provided.", nameof(payloadType));
+        }
+
+        PayloadType = payloadType;
+        Payload = payload;
+        PayloadContentType = payloadContentType;
+        DetachedPayload = detachedPayload;
+
+        var normalised = signatures?.ToArray() ?? Array.Empty<DsseSignature>();
+        if (normalised.Length == 0)
+        {
+            throw new ArgumentException("At least one signature must be supplied.", nameof(signatures));
+        }
+
+        // Deterministic ordering (keyid asc, signature asc) for canonical output.
+        Signatures = normalised
+            .OrderBy(static x => x.KeyId ?? string.Empty, StringComparer.Ordinal)
+            .ThenBy(static x => x.Signature, StringComparer.Ordinal)
+            .ToArray();
+    }
+
+    public string PayloadType { get; }
+
+    public ReadOnlyMemory<byte> Payload { get; }
+
+    public string? PayloadContentType { get; }
+
+    public IReadOnlyList<DsseSignature> Signatures { get; }
+
+    public DsseDetachedPayloadReference? DetachedPayload { get; }
+}