chore: Make AddFutureAwaitResult an AnyVal

[He-Pin]

Motivation:
less allocation.

Result:
less allocation

Note: The code it's self seems only been used in tests.

[pjfanning]

mima issue for scala 3 build

[error]  * static method AddFutureAwaitResult(scala.concurrent.Future)org.apache.pekko.http.impl.util.package#AddFutureAwaitResult in class org.apache.pekko.http.impl.util.package has a different result type in current version, where it is scala.concurrent.Future rather than org.apache.pekko.http.impl.util.package#AddFutureAwaitResult
[error]    filter with: ProblemFilters.exclude[IncompatibleResultTypeProblem]("org.apache.pekko.http.impl.util.package.AddFutureAwaitResult")

[raboof]

mima issue for scala 3 build

[error]  * static method AddFutureAwaitResult(scala.concurrent.Future)org.apache.pekko.http.impl.util.package#AddFutureAwaitResult in class org.apache.pekko.http.impl.util.package has a different result type in current version, where it is scala.concurrent.Future rather than org.apache.pekko.http.impl.util.package#AddFutureAwaitResult
[error]    filter with: ProblemFilters.exclude[IncompatibleResultTypeProblem]("org.apache.pekko.http.impl.util.package.AddFutureAwaitResult")

that is weird, but as this class is private[http] can be safely ignored AFAICS