Add comprehensive security scanning workflows for .NET SDK

[lukeina2z]

This commit implements complete security scanning for aws-xray-sdk-dotnet:

CodeQL Security Analysis (.github/workflows/codeql-analysis.yml)

• CodeQL analysis for C# code security scanning with security-extended queries
• Native .NET vulnerability scanning using 'dotnet list package --vulnerable'
• security-scan .NET global tool for .NET-specific security analysis
• Semgrep for pattern-based security analysis
• Uses commit hashes instead of version tags for supply chain security
• Runs on PR/push and weekly schedule
• .NET 6.0 support matching project requirements

Daily Security Scan (.github/workflows/daily-scan.yml)

• Scans published NuGet packages from NuGet.org twice daily
• Monitors core, aspnetcore, aspnet, awssdk, sqlserver packages separately
• Detects new vulnerabilities in existing published packages
• Downloads and analyzes actual NuGet packages (.nupkg files)
• Comprehensive current dependency scanning with multiple tools
• Generates detailed summary reports with vulnerability counts

Key Features

• Comprehensive coverage: source code, dependencies, published NuGet packages
• .NET-focused: dotnet vulnerability scan, OWASP Dependency Check, security-scan, Semgrep
• NuGet-aware: Package download and analysis from NuGet.org
• Security-focused: commit hashes, proper permissions, categorized results
• Production-ready: scans actual published packages from NuGet registry
• Robust: proper timeouts, error handling, and comprehensive reporting
• Multi-tool approach: combines 4 different security scanners for maximum coverage
• Actionable: clear reporting and GitHub Security tab integration

Includes OWASP Dependency Check suppressions file for managing false positives.

Addresses the critical security gap where aws-xray-sdk-dotnet had no automated security scanning despite being critical infrastructure used in production.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-advanced-security]

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on this overview. Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out the documentation.