Use feature gates instead of ignore tag for samples

Using the \`\`\`ignore tag for samples renders docs with a warning that "this code is not tested".

Instead, we can feature gate the code in the sample, e.g.:

```rs
/// ```
/// # #[cfg(feature = "run_all_samples")]
/// # async fn sample() -> anyhow::Result<()> { ... }
/// ```
```

And build/run all the samples with `cargo test --doc --features run_all_samples`

https://github.com/googleapis/google-cloud-rust/blob/562640dd4d223edd11fd5839a7c9e0eedf146cdd/.gcb/scripts/test-ignored-samples.sh#L32

This also would let us reclaim the \`\`\`ignore tag for samples that are Rust code, but may or may not compile. Currently we use \`\`\`no_rust for this case as a workaround. Note that this is not a big win, as no public docs are affected. It just seems more correct to me.