Switch to code attributes for OpentelemetryPhoenix

[danschultzer]

Stacked on #431 which needs to get in first. Check the latest commit for the changes here.

This conforms closer to the attributes in 1.27 docs:https://github.com/open-telemetry/semantic-conventions/blob/v1.27.0/docs/attributes-registry/code.md

phoenix.plug and phoenix.action is actually just the module and function.

It does introduce a bunch of breaking changes so let me know if backwards compability is required. I can keep the old attributes, but not sure if we should just go with major release instead since there's a lot of things that are different now? (same question as in #430)

Changes

• phoenix.plug -> code.namespace
• phoenix.action -> code.function

Adds

• code.namespace to liveview spans
• code.function to liveview spans