feat(runner): fixtures_url standalone mode + GitHub artifact URL support

[skylenet]

What

Makes eest_fixtures.fixtures_url a standalone source (when github_release is unset), pointing at either:

• a release / plain .tar.gz URL, or
• a GitHub Actions artifact URL (github.com/<owner>/<repo>/actions/runs/<run>/artifacts/<id>) — downloaded via the GitHub API (needs runner.github_token), unzipped, and its **inner .tar.gz auto-extracted`.

fixtures_subdir points at the fixtures within; no genesis is fetched (stateful-engine fixtures boot from the snapshot datadir). When github_release is set, fixtures_url keeps its existing role as the release-URL override.

This lets you consume a benchmarkoor build artifact directly:

tests:
  source:
    eest_fixtures:
      fixtures_url: https://github.com/ethpandaops/benchmarkoor/actions/runs/28947560261/artifacts/8170387928
      fixtures_subdir: benchmarkoor-build-artifacts/eest-payloads/geth/blockchain_tests_stateful_engine
# runner.github_token / BENCHMARKOOR_RUNNER_GITHUB_TOKEN required for artifact URLs

…and any plain tarball URL (e.g. a benchmarkoor-tests release asset) works with no token.

How

• config: UseFixturesURL() (fixtures_url set & no github_release) as a new mode in validate(); no github_repo required for URL mode.
• executor: prepareFromURL → downloadFromURL auto-detects artifact-vs-plain via parseGitHubArtifactURL; artifact URLs reuse a new shared downloadArtifactZip (by owner/repo/id), plain URLs reuse downloadAndExtractTarball. The artifact-name mode now uses the same downloadArtifactZip (dedup).

Tests / docs

• TestParseGitHubArtifactURL (artifact vs release vs plain) + TestEESTFixturesSource_UseFixturesURL (mode + validation). go build/go test green (except the pre-existing macOS-only /proc/mounts schelk test); golangci-lint --new-from-rev=origin/master 0 issues.
• docs/configuration.md updated: fixtures_url description, github_repo now Yes*, mode note, and a standalone-URL example.