c++ meta platform functionality, subscriptions, new GI, update build.cs

[alfred316]

Description

Adding MetaXR Platform functionality in C++ as a component. Getting entitlement, userID and subscription context for user. Dynamically reads app id from DefaultEngine.ini file. Added game instance with MetaXR platform initialization.

Height Task ID(s) (If applicable): T-5856, T-6558, T-3188

Type of change

Note: delete the lines that are not applicable and check the boxes (add a capital "X" in the square brackets) for the type of change.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] This change requires a documentation update

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] Any dependent changes have been merged and published in downstream modules
• [x] I have assigned this PR to myself in GitHub
• [x] I have assigned and notified Reviewers for this PR