Add Group-wide secure variables - BE

What does this MR do?

This MR adds an ability to define group-wide variables (Currently, only project-wide variables are available). This MR is specifically focusing on BE change. FE change would be another MR.

This MR's proposal is

Are there points in the code the reviewer needs to double check?

Groups can be nested. That means we have to look up related group-wide variables recursively. The process should be efficient.

Why was this MR needed?

Requested at

Screenshots (if relevant)

TBD (For the first iteration, it'd be like

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

/cc @grzesiek

Edited by Shinya Maeda

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