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

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.

https://github.com/apache/incubator-pekko-http/pull/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 (https://github.com/softwaremill/tapir/issues/3114).

Should we start making these constructors (see https://github.com/apache/incubator-pekko-http/pull/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

Start with headers.scala and the constructor changes referenced in pull request 178, then review the Scala 3 compilation impact on pekko-connectors and Tapir. Done would require a decided constructor visibility and backport policy, with the affected integrations compiling under Scala 3.

Written by the indexing model from the issue text.