Rework Analytics initialization

[sc-nikolaoslazaridis]

• This PR follows the Contribution Guide
• Changelog updated
• Upgrade guide entry added

Description / Motivation

Introduce new initialization functionality for the SDK and use it for the analytics packages.

• Add initSitecore to core package to enable initialization
• Adapt analytics-core, events, personalize packages to the new init function
• Create adapters for proxy environments for analytics and personalize packages in the nextjs package
• Use the new init functions in both nextjs templates

Testing Details

• Unit Test Added
• Manual Test/Other (Please elaborate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[github-actions]

📦 Package Size and Test Coverage Report

Package	Base Size	PR Size	Δ Change	Base Coverage	PR Coverage	Δ Change
analytics-core	122.52 KB	74.58 KB	🔻 -47.94 KB	100.00%	100.00%	0.00%
cli	58.70 KB	58.70 KB	✅ 0.00 KB	69.73%	69.73%	0.00%
content	397.70 KB	397.69 KB	🔻 -0.01 KB	92.26%	92.26%	0.00%
core	95.15 KB	104.19 KB	🔺 +9.04 KB	92.12%	⚠️ N/A	⚠️ N/A
create-content-sdk-app	243.64 KB	244.22 KB	🔺 +0.58 KB	96.22%	96.22%	0.00%
events	76.91 KB	66.41 KB	🔻 -10.50 KB	98.40%	98.17%	-0.23%
nextjs	430.44 KB	449.96 KB	🔺 +19.52 KB	89.73%	90.18%	+0.45%
personalize	63.45 KB	64.98 KB	🔺 +1.53 KB	99.32%	100.00%	+0.68%
react	289.47 KB	289.47 KB	✅ 0.00 KB	93.63%	93.63%	0.00%
search	7.11 KB	7.11 KB	✅ 0.00 KB	98.57%	98.57%	0.00%
Total	—	—	🔻 -27.78 KB	—	—	—

[illiakovalenko]

This is a solid solution - great job!
Please see some review comments below

[illiakovalenko]

👍