chore(tvix/[ca]store): allow building without tonic-reflection

- rename the feature to `tonic-reflection` in both tvix-store and tvix-
   castore.
 - set it to disabled in tvix-castore by default
 - enable it in tvix-store, and pull in tvix-castore with the feature
   enabled.

Change-Id: Ie22833d85569502cae55812f6eeb17a9c15b9e2c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9472
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/castore/Cargo.toml

@@ -37,5 +37,5 @@ tempfile = "3.3.0"
 tonic-mock = { git = "https://github.com/brainrake/tonic-mock", branch = "bump-dependencies" }
 
 [features]
-default = ["reflection"]
-reflection = ["tonic-reflection"]
+default = []
+tonic-reflection = ["dep:tonic-reflection"]