feat: add multi-region endpoint resolution via Endpoint static class

[OMpawar-21]

Summary

• Adds Endpoint.GetContentstackEndpoint(region, service) static class
  (Contentstack.Management.Core.Endpoints) for dynamic CMA host resolution
  across all 7 supported regions — replaces hardcoded api.contentstack.io
• regions.json registry is auto-downloaded from CDN on first use and cached
  on disk next to the DLL; self-heals if missing — no setup required
• Scripts/refresh-region.cs is bundled inside the NuGet package via
  contentFiles + build/contentstack.management.csharp.targets; the
  script is automatically placed in the consumer's Scripts/ folder on first
  dotnet build and can be run anytime with dotnet run Scripts/refresh-region.cs

Usage

using Contentstack.Management.Core.Endpoints;

string host = Endpoint.GetContentstackEndpoint("eu", "contentManagement", omitHttps: true);
// → "eu-api.contentstack.com"

var client = new ContentstackClient(new ContentstackClientOptions
{
    Host      = host,
    Authtoken = "<MANAGEMENT_TOKEN>"
});

[github-actions]

🔒 Security Scan Results

ℹ️ Note: Only vulnerabilities with available fixes (upgrades or patches) are counted toward thresholds.

Check Type	Count (with fixes)	Without fixes	Threshold	Result
🔴 Critical Severity	0	0	10	✅ Passed
🟠 High Severity	0	0	25	✅ Passed
🟡 Medium Severity	0	0	500	✅ Passed
🔵 Low Severity	0	0	1000	✅ Passed

⏱️ SLA Breach Summary

✅ No SLA breaches detected. All vulnerabilities are within acceptable time thresholds.

Severity	Breaches (with fixes)	Breaches (no fixes)	SLA Threshold (with/no fixes)	Status
🔴 Critical	0	0	15 / 30 days	✅ Passed
🟠 High	0	0	30 / 120 days	✅ Passed
🟡 Medium	0	0	90 / 365 days	✅ Passed
🔵 Low	0	0	180 / 365 days	✅ Passed

✅ BUILD PASSED - All security checks passed