Support environment keyword with trigger keyword

This MR is built on top of Support environment predefined variables in job... (!128694 - merged)

What does this MR do and why?

This MR introduces a support for environment keyword to be combined with trigger keyword (a.k.a. downstream pipelines).

It's a very simple change that moving the environment config entry to the processable superclass, so that it supports both build and bridge job types. Nevertheless, good amount of integration tests are added so that we can verify that the feature works in various configurations.

All of the fundamental works have already been done in the previous MRs, which you can see in the main issue Support `environment` keyword with `trigger` ke... (#369061 - closed).

Screenshots or screen recordings

Peek_2023-08-22_15-35

How to set up and validate locally

1. Enable support_ci_environment_variables_in_job_rules feature flag.
2. Activate EEU license in the GitLab instance.
3. Create upstream and downstream projects.
4. Add following .gitlab-ci.yml in the downstream project.

   provisioning:
       script: export
       rules:
         - if: $CI_PIPELINE_SOURCE == "pipeline" && $UPSTREAM_ENVIRONMENT_ACTION == "start"
   
   teardown:
       script: export
       rules:
         - if: $CI_PIPELINE_SOURCE == "pipeline" && $UPSTREAM_ENVIRONMENT_ACTION == "stop"

5. Add following .gitlab-ci.yml in the upstream project.

   stages:
       - deploy
       - cleanup
   
   .downstream-deployment-pipeline:
       variables:
           UPSTREAM_PROJECT_ID: $CI_PROJECT_ID
           UPSTREAM_ENVIRONMENT_NAME: $CI_ENVIRONMENT_NAME
           UPSTREAM_ENVIRONMENT_URL: $CI_ENVIRONMENT_URL
           UPSTREAM_ENVIRONMENT_ACTION: $CI_ENVIRONMENT_ACTION
           UPSTREAM_ENVIRONMENT_TIER: $CI_ENVIRONMENT_TIER
       trigger:
           project: multi-project-deployment-pipelines/downstream-project
           branch: main
           strategy: depend
   
   deploy-review:
       stage: deploy
       extends: .downstream-deployment-pipeline
       environment:
           name: review/$CI_COMMIT_REF_SLUG
           on_stop: stop-review
   
   stop-review:
       stage: cleanup
       extends: .downstream-deployment-pipeline
       environment:
           name: review/$CI_COMMIT_REF_SLUG
           action: stop
       when: manual

6. Make sure the upstream pipeline runs correctly and triggers the downstream pipeline.
7. Make sure the downstream pipeline has deployment context such as UPSTREAM_ENVIRONMENT_NAME.

MR acceptance checklist

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