git: Extend config validation to allow URL keys

Fixes #4547 (closed)

Adds support for : (colon) and @ (ampersand) such that URL keys can be specified.

Testing (GDK)

Validate the issue

1. Add the offending section to the <gdk-root>/gitaly/gitaly.config.toml or <gdk-root>/gitaly/gitaly-0.praefect.toml (with praefect enabled).

   [[git.config]]
   key = "http.https://example.com.proxy"
   value = "http://proxyUsername:proxyPassword@proxy.server.com:port"

2. gdk restart gitaly
3. Visit any project on your GDK. E.g http://localhost:3000/flightjs/Flight - If it blows up 🎉

Check the fix

1. Set <gdk-root>/gitlab/GITALY_SERVER_VERSION to the latest commit ID on this MR.
2. gdk reconfigure
3. Add the offending section to the <gdk-root>/gitaly/gitaly.config.toml (reconfigure overwrites this config)

   [[git.config]]
   key = "http.https://example.com.proxy"
   value = "http://proxyUsername:proxyPassword@proxy.server.com:port"

4. gdk restart gitaly
5. Visit any project on your GDK. E.g http://localhost:3000/flightjs/Flight - If you see listed files it worked 🎉