[benchmarker] Refactor congested_area.jsonnet into reusable components

[BenjaminPelletier]

In preparation for creating two more benchmarker scenarios similar to congested_area.jsonnet (#1724), it would be advantageous to factor our reusable elements of congested_area.jsonnet so we don't have very large content sections manually repeated between configurations. This PR attempts to do that, as well as aligning on S2 cell characteristics. That is, congested_area is intended to make everything happen within a single S2 cell. These changes also leverage recent S2 functions to make this the case, and present areas of interest in terms of the inscribed LatLngRect for the S2 cell selected.

A slightly-older version of this PR yielded the graph below with the settings below.

export INTRA_USS_NETEM_CONF="delay 600us 40us 25% distribution normal loss 0.0005%"
export INTER_USS_NETEM_CONF="delay 19ms 0.8ms loss 0.001%"
export NUM_USS=3
export NUM_NODES=3
export DSS_IMAGE="interuss/dss:v0.23.0"
export CORE_SERVICE_EXTRA_FLAGS="--enable_time_based_notification_index"