Feat: add grid view issue feature flag

What does this MR do and why?

This MR is used to add a feature flag called issues_grid_view that controls whether to enable the grid view for issues page. This feature is only used on the issue pages of groups and projects.

Note: this MR only adds the feature flag, and the implementation details can be found in this MR: !113012 (closed)

related issue: #323095 (comment 1271872665)

Follow-up MR: !113012 (closed)

/cc @JeremyWuuuuu @mtan-gitlab

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