Added the licenced `explain_code` flag

What does this MR do and why?

The MR repurposes the existing explain_code feature flag to be used as a licensed feature flag to protect the efforts delivered in the course of "Explain Code" efforts. In the same go, we re-name the old flag to explain_code_snippet to highlight that it is only to protect the select-and-explain functionality. The latter is scoped to user

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #403727