Support MSC3916 by adding a federation `/download` endpoint #1158

Job	Run time
changes	4s
changes	4s
check-lockfile	5s
check-lockfile	5s
lint-crlf	5s
lint-crlf	5s
lint-newsfile	39s
lint-newsfile	39s
check-schema-delta	37s
check-schema-delta	37s
lint	47s
lint	47s
check-sampleconfig	40s
check-sampleconfig	40s
Typechecking	1m 23s
Typechecking	1m 23s
lint-pydantic	45s
lint-pydantic	45s
lint-clippy	0s
lint-clippy	0s
lint-clippy-nightly	0s
lint-clippy-nightly	0s
lint-rustfmt	0s
lint-rustfmt	0s
linting-done	8s
linting-done	8s
calculate-test-jobs	8s
calculate-test-jobs	8s
trial-olddeps	6m 53s
trial-olddeps	6m 53s
portdb (3.8, 11)	2m 54s
portdb (3.8, 11)	2m 54s
complement (monolith, SQLite)	12m 28s
complement (monolith, SQLite)	12m 28s
portdb (3.11, 15)	2m 45s
portdb (3.11, 15)	2m 45s
trial-pypy	0s
trial-pypy	0s
complement (monolith, Postgres)	13m 20s
complement (monolith, Postgres)	13m 20s
complement (workers, Postgres)	30m 29s
complement (workers, Postgres)	31m 14s
cargo-bench	0s
cargo-bench	0s
cargo-test	0s
cargo-test	0s
trial (3.8, sqlite, all)	4m 12s
trial (3.8, sqlite, all)	4m 12s
sytest (focal)	8m 33s
sytest (focal)	8m 33s
sytest (focal, postgres)	10m 40s
sytest (focal, postgres)	10m 40s
trial (3.8, postgres, 11, all)	10m 11s
trial (3.8, postgres, 11, all)	10m 11s
trial (3.8, sqlite)	3m 57s
trial (3.8, sqlite)	3m 57s
sytest (focal, multi-postgres, workers)	13m 45s
sytest (focal, multi-postgres, workers)	13m 45s
sytest (focal, multi-postgres, workers, asyncio)	19m 42s
sytest (focal, multi-postgres, workers, asyncio)	19m 42s
export-data	2m 9s
export-data	2m 9s
tests-done	3s
tests-done	3s
	4h 55m 29s

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