This adds a BlobService trait, and an implementation for it using sled,
and one using a HashMap.
Change-Id: Id6bc1b629195d0b26fc503bd7d2dc9e43c41c317
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8087
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
