[GitHubActions] generator: arbitrary extra with: inputs on the checkout step

[avidenic]

Closes #389.

Adds string[] CheckoutWith to GitHubActionsAttribute — an escape hatch for actions/checkout inputs the typed knobs don't cover (token, ssh-key, path, clean, persist-credentials, sparse-checkout, set-safe-directory). Each entry is emitted verbatim inside the checkout step's with: block, after the typed keys, in supply order.

[GitHubActions("build",
    GitHubActionsImage.UbuntuLatest,
    InvokedTargets = [nameof(Compile)],
    FetchDepth = 0,
    CheckoutWith = ["token: ${{ secrets.CI_PAT }}", "path: src", "persist-credentials: false"])]

- uses: actions/checkout@v6
  with:
    fetch-depth: 0
    token: ${{ secrets.CI_PAT }}
    path: src
    persist-credentials: false

• Typed keys (submodules, lfs, fetch-depth, progress, filter, ref/repository) always emit first; CheckoutWith lines follow, in order.
• Raw passthrough, no validation — the build author owns the YAML. This also lets multi-line block scalars work (e.g. sparse-checkout: | with continuation lines as further entries), which a KEY: value validator would reject.
• Defaults to new string[0] and emits nothing when unset — existing workflows are byte-for-byte unchanged.

Scoped out (per the issue): the actions/checkout@v6 version override (clean additive follow-up) and post-checkout shell steps (a general step-injection mechanism, not this issue).

Tests — three Verify snapshot cases: checkout-with (typed key + the issue's example, proving ordering), checkout-with-only (guard opens with: with no typed key), checkout-with-sparse (multi-line block scalar — the case raw passthrough exists to support). No *ValidationTest.cs: the feature never throws, and the repo reserves those for validating features.

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⚠️ CI note: Test is green (all test projects pass). Pack currently fails on main because CHANGELOG.md was deleted (c2cdf60) while IPack/IHasChangelog still requires it — pre-existing base-branch breakage, unrelated to this PR.