Rollup merge of #135391 - jieyouxu:conditional-tracing, r=onur-ozkan

bootstrap: Implement conditional `tracing` infra

Add a conditional `tracing` setup that is gated behind `BOOTSTRAP_TRACING` env var. This `tracing` infra is implemented by:

- Introducing an optional `tracing` cargo feature in bootstrap.
- Added optional `tracing*` dependencies which are gated behind the `tracing` cargo feature.
- When `BOOTSTRAP_TRACING` is set, `bootstrap.py` will build bootstrap with `--features=tracing`.

There is a small trick here to share `BOOTSTRAP_TRACING` env var without having to add a separate env var:

- `BOOTSTRAP_TRACING=1` is not a registered `tracing` filter target, so that can be used to enable the `tracing` cargo feature yet not actually enable any tracing logs (useful for editor r-a setups without actually outputting any tracing logs).
- `BOOTSTRAP_TRACING=TRACE` and such are actually valid `tracing` filters, but that sets `BOOTSTRAP_TRACING` anyway.

Example usage: https://github.com/rust-lang/rust/pull/135299 (that experimental PR is not conditionally gated)
This PR is intentionally kept minimal to focus on the infra itself. To get actual mileage, instrumentations will need to be added to individual `Step`s and such.

r? `@onur-ozkan` (or reroll)

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