Add Command []string to PluginConfig and a new x/plugin package that
spawns plugin subprocesses (deduplicated by argv) and terminates them
on Shutdown. Linux uses Pdeathsig, Windows a kill-on-close job object,
so children follow an abrupt parent exit.
Wire Spawn into every config/parsing plugin constructor.
tokenTransport now delegates CloseIdleConnections to the underlying
*http.Transport so http.Client.CloseIdleConnections() works through
the wrapper. Three plugin Close() methods updated to unwrap via
HTTPClientTransport before calling CloseIdleConnections.
RoundTrip now short-circuits when caller already set Authorization,
avoiding an unnecessary req.Clone allocation on the pass-through path.
Add godoc comments for all 62 exported symbols across 20 files in
config/parsing/: 23 MDKey constants, 35 Parse*/List/Default* functions,
3 TLS helpers, 1 Args struct with 7 fields, and the package doc.
ginuerzh