Logs API/SDK - Move LogRecord implementation details from API to SDK

[lalitb]

As of now, the Logs API contains the implementation details for creation of LogRecord object. This should be ideally moved to SDK. The API should have minimal implementation details and storage allocation, and hence minimal performance overhead. Anyone wanting to create custom logs SDK will otherwise have to bear the baggage of these allocations happening in API.

This is also how trace signal is implemented, API has a Span trait, and it's Noop implementation, while the implementation details are in the Span structure in SDK.