Support specifying a path for custom endpoints

[cfbao]

Some apps' best entrypoint URL isn't the root path, but some path below that, e.g. http://localhost:12345/swagger.

If using AddProject, we can now get the Aspire dashboard to show a custom path by specifying appropriate properties in launchSettings.json. However, for executable resources with custom endpoints (added using WithEndpoint), there's no way for the dashboard to show a custom path, because EndpointAnnotation doesn't support that.

It'd be nice to support specifying a path for custom endpoints, and have the dashboard display that to be more helpful.