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/Cargo.lock

@@ -2458,9 +2458,9 @@ dependencies = [
 
 [[package]]
 name = "tokio-util"
-version = "0.7.4"
+version = "0.7.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0bb2e075f03b3d66d8d8785356224ba688d2906a371015e225beeb65ca92c740"
+checksum = "5427d89453009325de0d8f342c9490009f76e999cb7672d77e46267448f7e6b2"
 dependencies = [
  "bytes",
  "futures-core",
@@ -2804,6 +2804,7 @@ dependencies = [
  "thiserror",
  "tokio",
  "tokio-stream",
+ "tokio-util",
  "tonic",
  "tonic-build",
  "tonic-mock",