Re-introduce federation /download endpoint #1908

Job	Run time
lint-crlf	5s
changes	2s
check-lockfile	6s
lint-newsfile	1m 41s
check-sampleconfig	42s
check-schema-delta	39s
lint	3m 49s
lint-clippy	0s
lint-clippy-nightly	0s
Typechecking	1m 23s
lint-pydantic	42s
lint-rustfmt	0s
linting-done	2s
trial-olddeps	7m 10s
portdb (3.8, 11)	2m 52s
calculate-test-jobs	6s
portdb (3.11, 15)	2m 56s
cargo-bench	0s
cargo-test	0s
complement (monolith, SQLite)	13m 11s
complement (monolith, Postgres)	13m 39s
trial-pypy	0s
complement (workers, Postgres)	21m 16s
trial (3.8, sqlite, all)	4m 54s
sytest (focal)	10m 23s
trial (3.8, postgres, 11, all)	11m 16s
sytest (focal, postgres)	10m 20s
trial (3.8, sqlite)	4m 20s
sytest (focal, multi-postgres, workers)	13m 25s
sytest (focal, multi-postgres, workers, asyncio)	19m 16s
export-data	2m 12s
tests-done	4s
	2h 26m 31s

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