[plog_sync] Make signedsource crate public

Summary:
## Context

This tool will run locally before submitting diffs adding new PLog configs or making changes to existing ones to ensure basic rules are passed (oncall is valid) and we are consistent with the server view (expired configs can't be locally re-enabled).

## This diff

There's a really nice, self-contained library that's about 60 SLOCs that I'd hate to reimplement: https://www.internalfb.com/code/fbsource/fbcode/antlir/signedsource/src/lib.rs

Opportunistically making it `PUBLIC`.

Test Plan: _eyes

Differential Revision: D85798035

fbshipit-source-id: b9237c3f16f0dcb36c0117478963c441a06e25ae

Author: passy

antlir/signedsource/BUCK

@@ -5,6 +5,7 @@ oncall("antlir")
 rust_library(
     name = "signedsource",
     srcs = glob(["src/**/*.rs"]),
+    visibility = ["PUBLIC"],
     deps = [
         "md-5",
         "thiserror",