headers.scala case classes have package private constructors - Scala 3 enforces this

[pjfanning]

For some reason, Scala 2 seems to allow public access to package private constructors but Scala 3 enforces it.

We also had to relax the scope on 2 constructors to allow pekko-connectors to compile with Scala 3.

#178

It turns out that Tapir also uses these exact same constructors but Tapir is in its own package. Tapir recently added Pekko HTTP support but didn't add Scala 3 support for Pekko HTTP (softwaremill/tapir#3114).

Should we start making these constructors (see #178) public?

If so, should we backport to 1.0.1 even though this will probably affect binary compatibility?

Adding a hacky org.apache.pekko scoped util class to Tapir should allow a workaround that can create Content-Length/Content-Type instances.

wdyt? @jrudolph @mdedetrich

[pjfanning]

Looks like the Tapir team have worked around this issue so that means we don't need to make an immediate change - but it might still be worth considering whether to make the constructors public.

[jrudolph]

We shouldn't just make something public that was made private intentionally (i.e. probably because we want to uphold some invariants). That said, if there's a need for a public API, we can look into what's needed and add that.