Add Nix flake for mcproxyctl

Vendor dependencies and expose mcproxyctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/github.com/oschwald/maxminddb-golang/README.md

@@ -0,0 +1,36 @@
+# MaxMind DB Reader for Go #
+
+[![GoDoc](https://godoc.org/github.com/oschwald/maxminddb-golang?status.svg)](https://godoc.org/github.com/oschwald/maxminddb-golang)
+
+This is a Go reader for the MaxMind DB format. Although this can be used to
+read [GeoLite2](http://dev.maxmind.com/geoip/geoip2/geolite2/) and
+[GeoIP2](https://www.maxmind.com/en/geoip2-databases) databases,
+[geoip2](https://github.com/oschwald/geoip2-golang) provides a higher-level
+API for doing so.
+
+This is not an official MaxMind API.
+
+## Installation ##
+
+```
+go get github.com/oschwald/maxminddb-golang
+```
+
+## Usage ##
+
+[See GoDoc](http://godoc.org/github.com/oschwald/maxminddb-golang) for
+documentation and examples.
+
+## Examples ##
+
+See [GoDoc](http://godoc.org/github.com/oschwald/maxminddb-golang) or
+`example_test.go` for examples.
+
+## Contributing ##
+
+Contributions welcome! Please fork the repository and open a pull request
+with your changes.
+
+## License ##
+
+This is free software, licensed under the ISC License.