feat: add multi-region URL resolver and bump version to 1.6.0

[OMpawar-21]

Summary

• Adds Endpoint class for dynamic, region-aware URL resolution backed by a bundled regions.json registry
• Adds Utils.get_contentstack_endpoint() proxy so existing Utils import paths keep working without change
• Adds scripts/refresh_regions.py to pull the latest registry from Contentstack CDN
• Adds 53 tests covering all regions, aliases, services, error cases, and cache behaviour
• Bumps version to 1.6.0

Problem

Before this PR, the delivery host was either hardcoded (cdn.contentstack.io) or manually
constructed per region with string concatenation. There was no single authoritative source
for which URL maps to which region and service across the Python SDK.

Solution

A bundled regions.json (fetched live from artifacts.contentstack.com) is the single
source of truth. Endpoint.get_contentstack_endpoint() resolves any region + service
combination in O(n) lookup with zero HTTP calls at runtime.

from contentstack_utils import Endpoint

# Full URL
Endpoint.get_contentstack_endpoint("na", "contentDelivery")
# → "https://cdn.contentstack.io"

# Host only — pass directly to delivery SDK
host = Endpoint.get_contentstack_endpoint("eu", "contentDelivery", omit_https=True)
# → "eu-cdn.contentstack.com"
stack = contentstack.Stack(api_key, token, env, host=host)

# All endpoints for a region
Endpoint.get_contentstack_endpoint("azure-na")
# → {"contentDelivery": "...", "contentManagement": "...", ...18 keys}

[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	1	25	✅ Passed
🟡 Medium Severity	0	3	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

ℹ️ Vulnerabilities Without Available Fixes (Informational Only)

The following vulnerabilities were detected but do not have fixes available (no upgrade or patch). These are excluded from failure thresholds:

• Critical without fixes: 0
• High without fixes: 1
• Medium without fixes: 3
• Low without fixes: 0

✅ BUILD PASSED - All security checks passed

[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	1	25	✅ Passed
🟡 Medium Severity	0	3	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

ℹ️ Vulnerabilities Without Available Fixes (Informational Only)

The following vulnerabilities were detected but do not have fixes available (no upgrade or patch). These are excluded from failure thresholds:

• Critical without fixes: 0
• High without fixes: 1
• Medium without fixes: 3
• Low without fixes: 0

✅ BUILD PASSED - All security checks passed

[sunil-lakshman]

LGTM