chain spec builder: add generate mode for authority keys (#3811)

* core: use trait object for genesis constructor

* chain-spec-builder: use structopt

* chain-spec-builder: add new command to generate authority keys

* chain-spec-builder: use ? in main

* chain-spec-builder: fix stored and printed suri from seed

* chain-spec-builder: add comment about created keystore folders

* chain-spec-builder: simplify file write