config,server: add upstream.ignorePaths (#869)

Add upstream.ignorePaths config to prevent model swaps for static-asset
requests made through the /upstream/<model>/<path> passthrough endpoint.

- add UpstreamConfig with compiled *regexp.Regexp slice; invalid regex
returns an error at load time
- apply a default pattern matching common static-asset suffixes
(.js/.json/.css/.png/.gif/.jpg/.jpeg/.ico/.txt) when unset
- in handleUpstream, return 409 Conflict when a path matches and the
local model is not already loaded; peer and already-loaded models fall
through to normal dispatch
- update config-schema.json and config.example.yaml

Updates discussion: #868

internal/config/config_posix_test.go

@@ -266,6 +266,9 @@ groups:
 			"mthree":    "model3",
 		},
 		Groups: expectedGroups,
+		Upstream: UpstreamConfig{
+			IgnorePaths: DefaultUpstreamIgnorePaths(),
+		},
 		Routing: RoutingConfig{
 			Router: RouterConfig{
 				Use: "group",