Add endpoints {directory,snapshot}_get_partition_sha1_git and {revision,release}_get_partition

Similar to content_get_partition, but for these extra object types.

For directory and snapshot, we only return ids instead of whole objects because they are usually quite big and need to be paginated using other endpoints.

Resolves #4672 (closed)

Contains commits from !1022 (merged) !1023 (merged) !1025 (merged) !1024 (merged) (in that order)

Required for swh-scrubber#4685 (closed)