feat(social-media/posts): Add LinkedIn map

[jnv]

Description

Add LinkedIn map for social-media/posts profile. Currently this doesn't expose createdAt because provider returns that as Unix timestamps and we have no easy way to convert those into ISO date (will be fixed once this feature is exposed in OneSDK).

I have opted out of shared tests to use a fixed profile ID and add test for pagination (this could be consolidated separately).

Motivation and Context

Types of changes

• [ ] 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 change)

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] I haven't repeated the code. (DRY)
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[jnv]

@janhalama Thanks for the review, I have worked in your suggestions and expanded the tests a bit.