Consider using .json suffix for the materialized config

[doron-cohen]

Is your feature request related to a problem? Please describe.
.materialized_JSON suffix makes it harder for IDEs, GitHub etc to detect the file type.

Describe the solution you'd like
I suggest changing it to .materialized.json.

[smintz]

most IDEs, and even GitHub can be configured. I do see the point of using an extension that will be recognized by default by other tools. I do need to think about the migration plan for this. (probably better to do it now)