feat(tvix/store): add --json arg

This configures logging as JSON.

Change-Id: I22cdda84de215bfceda4e9d47bc8d487a5451a6e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8130
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/Cargo.lock

@@ -2683,6 +2683,16 @@ dependencies = [
  "tracing-core",
 ]
 
+[[package]]
+name = "tracing-serde"
+version = "0.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bc6b213177105856957181934e4920de57730fc69bf42c37ee5bb664d406d9e1"
+dependencies = [
+ "serde",
+ "tracing-core",
+]
+
 [[package]]
 name = "tracing-subscriber"
 version = "0.3.16"
@@ -2690,11 +2700,14 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a6176eae26dd70d0c919749377897b54a9276bd7061339665dd68777926b5a70"
 dependencies = [
  "nu-ansi-term",
+ "serde",
+ "serde_json",
  "sharded-slab",
  "smallvec",
  "thread_local",
  "tracing-core",
  "tracing-log",
+ "tracing-serde",
 ]
 
 [[package]]