capture: add support for the /batch request shape

[xvello]

Can be reviewed commit by commit as some code is moved around before being modified

• Mark the existing endpoint code v0 with goal of being as django-compatible as reasonable. We should then start a separate v1 endpoint with less debt. Move some code around to make navigation easier
• Increase trace metadata for better debugging when we mirror traffic
• Add a node-js /batch request as test fixture and make it pass. Note that the node SDK has a second form submission type that uses lz compression 🤦 but they seem to be going to /e/ anyway, not /batch
  • Keep the RawRequest instead of directly extracting events from it
  • Allow to get the token, sent_at and is_historical from the RawRequest body, to support the /batch fields. The is_historical might morph into a more generic data_type metadata down the road?
  • for now only covered by the django_compat.rs, will add end2end tests when I implement the is_historical handling in another PR
• There's still TODOs (routing to historical topic, specifying offset), but good enough to start mirroring traffic and checking errors
• Accept traffic on the /batch and /e endpoints, others will be added later on

[tomasfarias]

lgtm to start mirroring traffic 👍