feat(social-media/publish-post): Add LinkedIn

[jnv]

Description

This adds publish-post map for LinkedIn. Links and images upload are supported.

Example posts published by the test suite (visible in the snapshot):

• https://www.linkedin.com/feed/update/urn:li:activity:6905136788935901185
• https://www.linkedin.com/feed/update/urn:li:activity:6905136744518225921
• https://www.linkedin.com/feed/update/urn:li:activity:6905136774872383488
• https://www.linkedin.com/feed/update/urn:li:activity:6905136740177121281

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.