[Buildkite] Add tests to check elastic-package with serverless projects

[mrodm]

Currently, there is no step in CI to check about elastic-package creating Serverless projects or running system/pipeline/asset/static tests with it.

There is a daily job in the integrations repository where elastic-package test the packages using Serverless projects:

• Pipeline definition
• Buildkite link

However, this pipeline tests the latest elastic-package release merged into integrations. It does not test the latest changes in elastic-package before being released.

It would be interesting to have dedicated steps to check that elastic-package CLI can create a Serverless project and run the given tests with some package.

To be considered:

• Run tests with Observability and Security or just one project type?
• Create a new daily job or add a new step in the main pipeline?
  • Adding an step in the main pipeline would do this test in every Pull Request and every merge to main.
• Test using also different configurations (enabled Logstash? Add logstash to serverless provider #1645)

Related issues:

• Automate tests in CI for serverless packages #1451
• Add logstash to serverless provider #1645