Implement the `--validate` flag

[rnijveld]

This PR implements the validate flag, fixing #395

Aside from that there are a few changes to the sudo pipeline to allow for easier composability, maybe even (in the future) reusing large parts for su. I still think that the run and run_validate look an awful lot like each other, but did not want to spend any more time on refactoring them right now. I've also derived Debug on the Output struct in the validation framework, that allowed me some easy debugging of a failing test.

[github-actions[bot]]

Number of dependencies and binary size impact report

Metric	main	PR #612	Delta
Direct dependencies	5	5	-
Total dependencies	10	10	-
Binary size	976.3 KiB	985 KiB	+0.9%
Text size	566.9 KiB	569.9 KiB	+0.5%

Dependencies diff

```diff └─ sudo-rs [v0.2.0-dev.20230627] ├─ glob [v0.3.1] ├─ libc [v0.2.147] ├─ log [v0.4.19] ├─ signal-hook [v0.3.15] | ├─ libc [v0.2.147] | ├─ signal-hook-registry [v1.4.1] | | └─ libc [v0.2.147] | └─ cc [v1.0.79] └─ signal-hook-registry [v1.4.1] ```

[codecov[bot]]

Codecov Report

Patch coverage: 84.36% and project coverage change: -0.35 :warning:

Comparison is base (3b01636) 86.20% compared to head (427fd82) 85.85%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #612 +/- ## ========================================== - Coverage 86.20% 85.85% -0.35% ========================================== Files 62 62 Lines 8341 8440 +99 ========================================== + Hits 7190 7246 +56 - Misses 1151 1194 +43 ``` | [Impacted Files](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/612?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety) | Coverage Δ | | |---|---|---| | [src/pam/error.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/612?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3BhbS9lcnJvci5ycw==) | `80.31% <0.00%> (-2.62%)` | :arrow_down: | | [src/sudo/pipeline.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/612?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3N1ZG8vcGlwZWxpbmUucnM=) | `84.00% <79.38%> (-9.45%)` | :arrow_down: | | [src/system/timestamp.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/612?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3N5c3RlbS90aW1lc3RhbXAucnM=) | `89.80% <80.95%> (-0.42%)` | :arrow_down: | | [src/pam/mod.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/612?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3BhbS9tb2QucnM=) | `91.18% <91.66%> (+0.95%)` | :arrow_up: | | [src/sudo/pam.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/612?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3N1ZG8vcGFtLnJz) | `96.03% <92.85%> (+7.71%)` | :arrow_up: | | [src/sudo/mod.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/612?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3N1ZG8vbW9kLnJz) | `79.12% <100.00%> (+1.59%)` | :arrow_up: | | [src/sudoers/policy.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/612?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3N1ZG9lcnMvcG9saWN5LnJz) | `93.40% <100.00%> (+0.54%)` | :arrow_up: | ... and [6 files with indirect coverage changes](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/612/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety)

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