Migrate CI from Travis to GitHub Actions (main branch)

[Copilot]

This updates main from Travis CI to GitHub Actions using the shared logstash-plugins/.ci@1.x reusable workflows, preserving the test coverage actually exercised by the resolved Travis config. For this plugin, Travis resolved to unit-test coverage only, so the migration adds only the unit workflow path.

• CI migration (Travis → GHA, parity-preserving)

  • Added .github/workflows/unit-tests.yml as a reusable-workflow caller to logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x.
  • Kept the required dual-job shape (tests + scheduled matrix fanout) with secrets: inherit on both jobs.
  • Added explicit workflow permissions:
    • contents: read.

• Workflow surface aligned to active branch model

  • Triggers configured for push, pull_request, workflow_dispatch, and daily schedule.
  • Scheduled matrix targets active branch set for this repo: [main].

• Repository cleanup

  • Removed .travis.yml.
  • Replaced Travis badge in README.md with the new GitHub Actions Unit Tests badge (?branch=main).

• Travis matrix → GHA mapping

  Resolved Travis signal	GHA workflow added
  Base test matrix (unit path)	.github/workflows/unit-tests.yml
  INTEGRATION=true present	none
  SECURE_INTEGRATION=true present	none
  HAS_PERFORMANCE_TESTS=1 / _performance enabled for this plugin	none

jobs:
  tests:
    uses: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
    secrets: inherit
    with:
      timeout-minutes: 60

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