[CI] Divide packages tested in serverless CI pipeline to be tested in batches

Currently, the CI Pipeline in charge of testing the packages using a Elastic Serverless project (https://buildkite.com/elastic/integrations-serverless) creates one Serverless project (Observability or Security) and tests all those packages with that project/stack sequentally.

Given the current number of packages (~325), this pipeline started to last 4 hours. And this execution time will always be larger with any new package added/created.

That caused that the timeout required to be increased up to 6 hours (https://github.com/elastic/integrations/pull/12998).

This pipeline should be revisited in order to try to reduce the total time spent testing the packages with an Elastic Serverless project.

Possible approach (any other?):
- Split the number of packages in different batches (batches 100, 150 or 200 packages).
- Create dynamically a new Buildkite step for each batch defined and each step:
- Creates a new Elastic Serverless project.
- Tests all the packages in that batch sequentially.
- Deletes the Elastic Serverless project.