fix(schema): add missing config properties accepted by code

[chad-loder]

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Fixes #2518 🦕

---

The config JSON schema (schemas/config.json) is missing several
properties that the TypeScript types in src/manifest.ts accept at
runtime. This causes false-positive validation errors for users who
validate their config against the published schema.

Added properties

Property	Type	TypeScript source
include-commit-authors	boolean	ReleaserConfigJson L183
component	string	ReleaserPackageConfig L227
package-name	string	ReleaserPackageConfig L226
skip-snapshot	boolean	ReleaserConfigJson L193

All four are added to the ReleaserConfigOptions definition and to
the top-level properties allowlist. Purely additive — no existing
properties are modified, narrowed, or removed.

Repro (before this fix)

cat > /tmp/rp-test.json <<'JSON'
{
  "release-type": "python",
  "include-commit-authors": true,
  "packages": { ".": { "package-name": "my-pkg" } }
}
JSON

npx check-jsonschema \
  --schemafile https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json \
  /tmp/rp-test.json
# => Additional properties are not allowed ('include-commit-authors' was unexpected)