feat: Create `phoenix.otel` package (#4230)

* Sketch out Phoenix OTel wrapper

* Add settings handler

* Add package boilerplate

* Improve protocol inference

* Add symlink to otel package for development

* Flesh out examples

* README update

* Add `register` convenience method

* Clean up type annotations

* Clean up type annotations

* Clean up and update documentation

* README and usability improvements

* Add header support

* Delete symlink

* Update manifest

* Update correct manifest

* Improve urlencoding fallback message

* Tweak ergonomics