Add support for private s3 mvn repos

[mikub]

Currently private s3 mvn repo support is achievable via following workaround requiring rebuild:
https://github.com/mikub/titanoboa/wiki/Server-Configuration#using-private-s3-maven-repositories

It would be nice if this would work out of the box, currently people have to maintain a separate build/fork to make this work.

[mikub]

Added new feature here: 1a3056b
Whenever s3-wagon-private dependency is found in ext-dependencies it is automatically initiated and registered in aether prior to other dependencies' retrieval. This way private s3 can be supported without including s3-wagon-private in project.clj by default (and potentially unnecessarily bloating titanoboa since most ppl dont need s3 private mvn repos).

Tested this with a sample private s3 mvn repo and by adding [s3-wagon-private "1.3.2" :exclusions [ch.qos.logback/logback-classic]] into ext-dependencies.

Also there seemed to be some issues with consistency of mvn metadata for [s3-wagon-private "1.3.4"], so I am sticking to 1.3.2 at the moment.