feat: add experiment key to variant and exposure event

[bgiori]

Summary

add experiment key to exposure event and variant

Checklist

• [x] Does your PR title have the correct title format?
• Does your PR have a breaking change?: no

[github-actions[bot]]

:tada: This PR is included in version 1.11.0 :tada:

The release is available on:

• GitHub release
• v1.11.0

Your semantic-release bot :package::rocket: