Add Sliding Sync `/sync/e2ee` endpoint for To-Device messages #1347

Job	Run time
check-lockfile	7s
lint-crlf	8s
changes	3s
lint-newsfile	35s
check-sampleconfig	56s
lint	54s
Typechecking	1m 31s
check-schema-delta	43s
lint-pydantic	47s
lint-rustfmt	0s
lint-clippy-nightly	0s
lint-clippy	0s
linting-done	4s
calculate-test-jobs	6s
trial-olddeps	3m 59s
complement (monolith, SQLite)	3m 58s
portdb (3.8, 11)	3m 1s
trial-pypy	0s
portdb (3.11, 15)	2m 48s
complement (monolith, Postgres)	3m 58s
complement (workers, Postgres)	3m 58s
cargo-bench	0s
cargo-test	0s
trial (3.8, sqlite, all)	3m 49s
trial (3.8, postgres, 11, all)	3m 48s
sytest (focal)	3m 35s
trial (3.8, sqlite)	3m 48s
sytest (focal, postgres)	3m 37s
sytest (focal, multi-postgres, workers)	3m 36s
sytest (focal, multi-postgres, workers, asyncio)	3m 38s
export-data	49s
tests-done	4s
	54m 20s

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