refactor(tvix/store): use read_all_and_chunk in gRPC blobservice

This was the last piece of code using BlobWriter.

We can also use `read_all_and_chunk`, it's just requires a bit more
plumbing:

 - The data coming from the client (stream) needs to be mapped (we
   extract the .data field).
 - The stream needs to be turned into an (async) reader
 - The reader needs to be made sync, and that code using the sync reader
   needs to be in a `task::spawn_blocking`.

Change-Id: I4e374e1a9f47d5a0933f59a8f5c121185a5f3e95
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8260
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/store/Cargo.toml

@@ -26,6 +26,7 @@ tonic = "0.8.2"
 tracing = "0.1.37"
 tracing-subscriber = { version = "0.3.16", features = ["json"] }
 walkdir = "2.3.2"
+tokio-util = { version = "0.7.7", features = ["io", "io-util"] }
 
 [dependencies.tonic-reflection]
 optional = true