RUM-5925: Add Log event JSON schema

[0xnm]

What does this PR do?

This PR add JSON schema for the Log event, it will be needed for the event mapping.

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• [ ] Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)