Add mscgen support (fixes #12)

[tmpgnh]

Message Sequence Chart (MSC) support is added using msc-generator [1] as suggested by [2]. The backend tool is installed into the server image from upstream Ubuntu, under the service endpoint "/mscgen". (Disclaimer: I'm packaging the tool for Debian.) Msc-generator's signalling chart part is heavily extended and completely rewritten from Michael C McTernan’s mscgen with a lot of enhancements; and also supports block charts and generic graphs (dot format) as of today.

Please consider this PR as an extension to the impressive list of supported backends. I'd be happy to receive any comments.

[1] https://gitlab.com/msc-generator/msc-generator [2] https://github.com/yuzutech/kroki/issues/12#issuecomment-1368139643