+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
To read it more like prose, we could have
```
  def provide = afterWord("provide")  
  "Java DSL" must provide {
    "Source" which {
      "allows creating the same Sources as Scala DSL" in {
        ...
```

The output would be
```
[info] DslFactoriesConsistencySpec:
[info] Java DSL
[info]   must provide
[info]     Source which
[info]     - allows creating the same Sources as Scala DSL
[info]       + Matched: Scala:apply(org.reactivestreams.Publisher) == Java:from(org.reactivestreams.Publisher)
...
```