Add taskchampion-sync-server-storage-postgres

This is built to be more robust than the SQLite storage, and to
integrate with other applications. The idea is that for example a web
application might interact with the same DB to create and delete clients
as customers come and go.

.github/workflows/checks.yml

@@ -106,7 +106,8 @@ jobs:
       - uses: obi1kenobi/cargo-semver-checks-action@v2
         with:
           # exclude the binary package from semver checks, since it is not published as a crate.
-          exclude: taskchampion-sync-server
+          # exclude postgres temporarily until it is released
+          exclude: taskchampion-sync-server,taskchampion-sync-server-storage-postgres
 
   mdbook:
     runs-on: ubuntu-latest