Spike: dynamically updated LB3 API (routes + spec)

[bajtos]

This is a follow-up for the spike #2318, see the discussion and PoC code for more context.

In LB3, it's possible to change the shape of REST API at runtime by adding or removing shared methods or even entire models.

The PoC version of LB3+LB4 does not support spec updates, we need to do a spike to find the best solution. (For example, mountExpressRouter can accept a spec getter instead of the spec object.)

As for the actual request handling, the PoC version reloads route handlers in "full" mode but does not reload in "router" mode. As part of the stories created from the spike, we should fix "router" mode to reload the LB3 handler when needed.

Related issues: #1849 #2389 #2391

[jannyHou]

time box the spike to one week.

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