Remove `FeatureFlags` access via `Global` singleton

[stefandunca]

Following on the discussion here #14590 improve on the current approach of exposing FeatureFlags nim instance through context properties and use an intermediate store for this.

[caybro]

@micieslak can't remember from the top of my head if we agreed on how to tackle this issue at the offsite :)

[Khushboo-dev-cpp]

adding a note that this needs to be done for swap too!