feat: add Attestation Chain and Triage Evidence API clients and models

- Implemented Attestation Chain API client with methods for verifying, fetching, and managing attestation chains.
- Created models for Attestation Chain, including DSSE envelope structures and verification results.
- Developed Triage Evidence API client for fetching finding evidence, including methods for evidence retrieval by CVE and component.
- Added models for Triage Evidence, encapsulating evidence responses, entry points, boundary proofs, and VEX evidence.
- Introduced mock implementations for both API clients to facilitate testing and development.

src/Scanner/StellaOps.Scanner.WebService/Program.cs

@@ -470,6 +470,7 @@ apiGroup.MapScanEndpoints(resolvedOptions.Api.ScansSegment);
 apiGroup.MapReachabilityDriftRootEndpoints();
 apiGroup.MapProofSpineEndpoints(resolvedOptions.Api.SpinesSegment, resolvedOptions.Api.ScansSegment);
 apiGroup.MapReplayEndpoints();
+apiGroup.MapWitnessEndpoints(); // Sprint: SPRINT_3700_0001_0001
 
 if (resolvedOptions.Features.EnablePolicyPreview)
 {