server info should include executable

[maartenbreddels]

Problem

It is useful for a kernel to know if it shares the same Python environment with the server. For ipywidgets and libraries we can do some automated checks to see if there a library is installed in a different environment as the server.

Proposed Solution

The server info contains the PID of the jupyter server, but getting the executable from that is platform dependant. If we would simply store sys.executable along with the rest of the information, that would be very helpful.

[kevin-bates]

This makes good sense to me @maartenbreddels - thanks for creating this issue. I don't see any compatibility issues with this. I suppose some external apps could read the server info files and be strict about "unknown" keys, but that would be extreme, and this field would be purely informational.

Will you be providing the PR once a general consensus has been met?