BGS: transport compression for non-trivial CAR files

[bnewbold]

Eg, if doing a sync method on a full repo download, should get HTTP transport compression. CAR files can get big, and compress quite well. IMO so well that we should do this at the BGS (app) layer, not at load-balancer level.

Might already even be doing this, for free! But need to check. In particular, mimetype "compressible" database might not be configured for the CAR mimetype.