feat(capture): Add historical_migration flag to batch capture

[bretthoerner]

Problem

Rather than (or in addition to) having to manually add/remove tokens to/from TOKENS_HISTORICAL_DATA, we could automatically opt in migration tools.

Changes

Add historical_migration flag to the batch endpoint, to be used by migration scripts in the future.

(Historical migrations should always use the batch endpoint.)

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

How did you test this code?

New and existing tests.

[bretthoerner]

you have a new capture companion too, correct?

No, new capture doesn't support batch yet.