Migrate CI from Travis to GitHub Actions (main branch)

[Copilot]

This PR migrates CI on main from Travis to GitHub Actions using the shared logstash-plugins/.ci@1.x reusable workflows, preserving the effective Travis test surface. It replaces Travis-specific wiring, updates status signaling in README, and retires Travis config from the branch.

• Travis matrix parity (resolved imports)

  • Resolved .travis.yml import chain (travis.yml → defaults.yml + matrix.yml + exec.yml) and mapped the effective matrix to GHA.
  • Effective Travis test type on this plugin was unit tests only (no INTEGRATION=true, no SECURE_INTEGRATION=true; performance gated by HAS_PERFORMANCE_TESTS=1 and not enabled here).
  • Result: added only the unit-test workflow caller.

• Workflow changes

  • Added: .github/workflows/unit-tests.yml
  • Calls: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
  • Includes required trigger/model shape:
    • push + pull_request on main
    • schedule with branch fan-out matrix ([main]) using ref: ${{ matrix.branch }}
    • workflow_dispatch
  • Includes secrets: inherit on both caller jobs.
  • Adds explicit least-privilege token permissions:
    • permissions: { contents: read }

• README badge migration

  • Replaced Travis badge with GitHub Actions Unit Tests badge pinned to ?branch=main.

• Travis retirement

  • Deleted .travis.yml.

jobs:
  tests:
    if: github.event_name != 'schedule'
    uses: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
    secrets: inherit
    with:
      timeout-minutes: 60

  scheduled:
    if: github.event_name == 'schedule'
    strategy:
      matrix:
        branch: [main]
    uses: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
    secrets: inherit
    with:
      ref: ${{ matrix.branch }}
      timeout-minutes: 60