Add Scala 3 support to build and CI matrix

[felixbr]

Hi, this is early work on supporting a native Scala 3 build for Finagle.

The changes I've made are close to the current setup in twitter/util (including my PR there for Scala 3 CI).

Right now this only adds Scala 3 to finagle-init as a PoC because other modules are still blocked on twitter/util as far as I know.

I'm aware that many Finagle modules are already usable with Scala 3 projects using the cross-compilation (= .cross(CrossVersion.for3Use2_13)) feature. This works decently well for applications but is not a suitable solution for libraries which build on Finagle or more complex cross-building setups.

Note: Currently this branches from release not from develop because the latter seems to be unstable. I will rebase onto develop before converting the draft into a real PR ready to be merged.

[xin301x]

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