Allow to compile akka-http module with Scala 3

[jchyb]

This mostly involves specifying types for implicit vals and adding correct parentheses in lambda types. Compilation with -source:3.0-migration was simple, requiring only passing some implicits explicitly in PathDirectives. When I removed the option, another slightly more severe issue popped up, with the compiler seemingly incorrectly inferring types used in javadsl/scaladsl mappings - because of that, the compiler was unable to find the correct implicit. This was fixed by manually specifying types in places like this. What's interesting, this only happened when going through a collection previously converted from java collection (via scala.collections.JavaConverters). This can be for example observed in Rejections.scala.

Nevertheless, now the 'akka-http' module compiles with both 3.1.1, 3.1.2-RC3 and 2.13.8 (2.12.15 errored at the akka-http-core module).

References #4079, task "Enable akka-http module for Scala 3 (currently disabled in Scala 3) and make main code compile"

[lightbend-cla-validator]

Hi @jchyb,

Thank you for your contribution! We really value the time you've taken to put this together.

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https://www.lightbend.com/contribute/cla

[jchyb]

I confirm I signed the Lightbend CLA

[jrudolph]

LGTM, nice first step.

I agree, removing -source:3.0-migration will be some work but it's good enough for me to get something working and incrementally improve on it. Btw. are there any compatibility problems with -source:3.0-migration or will users be able to use the library with and without also setting that flag? (I assume they will get into the same problems as we do when implicit resolution doesn't work quite as it works for Scala 2)

[jchyb]

About the compilation with -source:3.0-migration, I think it should still work for the users (at least the docs do not say otherwise), but it sounds safer to remove it if possible, especially that it usually should not be that difficult. Something that might not have been clear looking at my PR message, I did successfully remove the flag in the second commit of the PR (just for akka-http, not akka-http-core as it seems better to finish implementing parsers there first), it's just that I had to write explicit types in some weird places (likely a bug in the compiler, trying to minimize it now).