Epic: Prebuilds only available in user-specified regions

[atduarte]

Context

At the moment, prebuilds run in the US region which means that whenever someone starts a workspace in a different region, they must be transferred across continents. This results in slower workspace startup times and poor cost effectiveness.

The problem will be exacerbated as soon as we launch #7489.

Goal

Improve workspace startup times and cost effectiveness.

Proposed solution

Only allow prebuilds to be used in the regions that they were computed and stored, and require users to specify what regions those should be for each project. When nothing is specified, the prebuild will be computed, stored and usable only in the default user's region.

[stale]

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