[JENKINS-15198] Add support for Maven Toolchains files via config-file-p...

I added support based on the work done in file-provider-plugin.

Remarks:
1) it is based on the settings/global settings processing, however, there are some places (publisher, pom reader) where I did not add toolchains as I don't think this is necessary
2) I'm checking the version of the file provider plugin (at least 2.3) to switch to the proper mediator so it is not possible to test full support with the current version of the file provider plugin, but I tested it with test commented out
3) I did not support toolchains configuration based on a local file (but I don't think this is an issue as global settings don't)