generate warnings counts in PR comment

[Becheler]

Refs #496

Before submitting

• This PR targets the develop branch.
• I searched for an existing PR or issue covering the same change.
• My contribution is licensed under the Boost Software License 1.0.

Type of change

• Bug fix
• New feature or API addition
• Refactor (no behavior change)
• Documentation
• Build, CI, or tooling
• Other (specify below)

Does this PR introduce a breaking change?

• Yes (describe migration impact below)
• No

What this PR does

Adds a new git workflow to

1. Download the logs from develop (baseline) and the current PR
2. Compare warnings counts before/after
3. Trigger a bot to publish the count table before/after/delta in the PR conversation

Motivation

It's not trivial for contributors to have a clear vision of the impacts of their contributions on the total warning counts.

Testing

1. I made my fork branch the default
2. I tested it triggers on a louvain fix
3. the table is correctly published

See: Becheler#1 (comment)

That generates the following comment :

Compiler-warning counts vs `develop` (auto-generated).
PR run `27095071066` vs develop run `27060632166` (`21cd4df091`).

| Job | Baseline | After | Delta |
|-----|---------:|------:|------:|
| macos (clang, 14) | 75 | 48 | -27 |
| macos (clang, 17) | 70 | 43 | -27 |
| macos (clang, 20) | 70 | 43 | -27 |
| ubuntu (clang-19, 14) | 75 | 48 | -27 |
| ubuntu (clang-19, 17) | 75 | 48 | -27 |
| ubuntu (clang-19, 20) | 75 | 48 | -27 |
| ubuntu (clang-19, 23) | 75 | 48 | -27 |
| ubuntu (gcc-14, 14) | 51 | 33 | -18 |
| ubuntu (gcc-14, 17) | 51 | 33 | -18 |
| ubuntu (gcc-14, 20) | 51 | 33 | -18 |
| ubuntu (gcc-14, 23) | 51 | 33 | -18 |
| windows_msvc_14_3 (msvc-14.3) | 1226 | 1226 | 0 |

<!-- Sticky Pull Request Commentwarning-counts -->

Checklist

• Existing tests pass (b2 in the test/ directory).
• New behavior is covered by a test, or this is a docs / build / refactor change.
• Documentation was updated if user-facing behavior changed.
• No new compiler warnings on the platforms I built against.

[jeremy-murphy]

Looks awesome, can't wait to see it in action!