Draft: Adds environment keyword to processable entry class

What does this MR do and why?

Adds the environment sub-keyword to Ci::Entry::Bridge. Our docs mention the usage of this sub-keyword, but the linter was not updated.

Screenshots or screen recordings

Before	After

**Pipeline: **

Manual QA for testing variables in different environments

How to set up and validate locally

Run a pipeline with this config:

stages:
  - deploy
 
deploystacks:
  stage: deploy
  trigger:
    include: child-pipeline.yml
  parallel:
    matrix:
      - PROVIDER: aws
        STACK: [monitoring, app1, app2]
  environment: $PROVIDER/$STACK

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

#384376 (closed)