Changing the type of callback function in Export function to std::function

[DebajitDas]

Fixes # (issue)

Changes

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[ThomsonTan]

Wondering the reason behind this change, and does this break our SDK API?

[lalitb]

Wondering the reason behind this change, and does this break our SDK API?

This PR is not merged in the main, but a separate feature branch async-changes. As @owent and @DebajitDas are working simultaneously on this. Eventually, once the feature is complete, will raise a PR to the main for review. We need to ensure that the existing SDK API shouldn't change with this.