Updated code owners

[brendankowitz]

Description

This pull request updates the .github/CODEOWNERS file to provide more granular and clearly documented code ownership across feature verticals in the Microsoft FHIR Server repository. The changes ensure that the appropriate subject matter experts and teams are automatically assigned to review pull requests based on the affected areas of the codebase.

Ownership structure improvements:

• Replaced the previous global fallback owner with a comprehensive, feature-based ownership mapping, assigning specific owners to SQL Server, CosmosDB, API, security, bundle, validation, health, conformance, infrastructure, and core framework features.
• Added detailed comments and section headers to clarify the purpose and coverage of each ownership group, improving maintainability and transparency.
• Included documentation and configuration files in the ownership mapping, ensuring that changes to markdown and build files are reviewed by relevant contributors.

FHIR Team Checklist

• Update the title of the PR to be succinct and less than 65 characters
• Add a milestone to the PR for the sprint that it is merged (i.e. add S47)
• Tag the PR with the type of update: Bug, Build, Dependencies, Enhancement, New-Feature or Documentation
• Tag the PR with Open source, Azure API for FHIR (CosmosDB or common code) or Azure Healthcare APIs (SQL or common code) to specify where this change is intended to be released.
• Tag the PR with Schema Version backward compatible or Schema Version backward incompatible or Schema Version unchanged if this adds or updates Sql script which is/is not backward compatible with the code.
• When changing or adding behavior, if your code modifies the system design or changes design assumptions, please create and include an ADR.
• CI is green before merge
• Review squash-merge requirements

Semver Change (docs)

Patch|Skip|Feature|Breaking (reason)