Fixes for orchestrator doc hiddens and re-exports (#2801)

## Motivation and Context
This PR:
- Makes the `#[doc(hidden)]` attributes on orchestrator-only config
functions be conditional based on whether the orchestrator is default or
not.
- Adds re-exports specific to the orchestrator.
- Centralizes `BoxError`.
- Changes organization of interceptor context types a bit.
- Reduces visibility of `Phase` to `pub(crate)`.

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._