[JENKINS-66563] add a supported API for a plugin to insert jars into its classpath (#5711)

* Revive support for `URLClassLoader`

* [JENKINS-66563] add a supported API for a plugin to insert jars into its classpath

Required for BouncyCastle plugin so it does not try and expose its
classes when running in a JVM in FIPS mode, but can be other use cases
as yet unknown.

* [JENKINS-66563] add test case for new functionality

* [JENKINS-66563] and @since javadoc

* [JENKINS-66563] correct indent for new test

* mark the API Beta and document do not do this

Feedback from Jesse was we should be clearer that you probably should
not be using this interface and that there are other approaches that
should be used first.
To that end add some extra Javadoc and mark the API as Beta

* [JENKINS-66563] fix javadoc errors

* Fix typo

Co-authored-by: Devin Nusbaum <dwnusbaum@users.noreply.github.com>

Co-authored-by: Basil Crow <me@basilcrow.com>
Co-authored-by: Tim Jacomb <timjacomb1+github@gmail.com>
Co-authored-by: Tim Jacomb <21194782+timja@users.noreply.github.com>
Co-authored-by: Devin Nusbaum <dwnusbaum@users.noreply.github.com>