Add ability to provide a custom logger class to DiskANN index

[d4mator]

• Does this PR have a descriptive title that could go in our release notes?
• Does this PR add any new dependencies?
• Does this PR modify any existing APIs?
  • Is the change to the API backwards compatible?
• Should this result in any changes to our documentation, either updating existing docs or adding new ones?

Reference Issues/PRs

What does this implement/fix? Briefly explain your changes.

DiskANN code output various statements to cout and cerr streams. In some cases this code runs as part of the service that cannot use these streams, such as when run as an Autopilot service. This output is crucial for monitoring, debugging.

The change adds an interface for the logging component and allows to specify a custom implementation of the logger. By default logging is done to a console implementation (similar to the existing cout and cerr).

Any other comments?

Mostly converted cout and cerr statements from index.cpp, but not everything. Only the most important parts were converted.