feat: use HARRemix and record-har by default

[runspired]

Currently the default is to serve assets using ember s. We should capture the output from these servers as a HAR using tracerbench record-har then serve them back up using HARRemix (later tracerbench-serve see #2 ). This will eliminate potential noise from the network / backend APIs providing a more stable and reproducible benchmark.

Users requiring API auth / this level of stability today can customize their build and serve commands to achieve this.

[lifeart]

related issue about documentation examples: TracerBench/tracerbench#340