+str #16114 DSL consistency (factories)

Adds consistency specs for static methods (Source.from / etc).
Added all missing methods.

Depends on my previous PR, please Ignore the first commit - only review https://github.com/ktoso/akka/commit/d1508bc803c195d171829b32854cf51794afc949 this one.

Resolves #16114
Leftover?