Provide ability to reuse results of variable expressions in if directives

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Proposal

Provide ability to reuse results of expressions involving variables in rules:if directives. That is, provide the ability to assign the result of an expression to a variable/macro so that instead of reusing the whole expression again and again in multiple if directives, that variable/macro can be used.

To have them not get polluted by env variables inadvertently, maybe use a different syntax than ${name} - something like %{name}.

Consider the below snippet as an example. An omnibus-gitlab package build is considered an EE build if either of the following are true

1. ee variable is true
2. branch name ends with -ee
3. tag name consists of ee
4. It is an auto-deploy branch
5. It is an auto-deploy tag

triggered-ee-job is supposed to run only on EE builds that are triggered. Similarly, scheduled-ee-job is supposed to run only on EE builds that are scheduled.

macros:
  EE_BUILD: '$ee == "true" || $CI_BRANCH_NAME =~ /^.*-ee$/ || $CI_BRANCH_NAME =~ /^[0-9]+-[0-9]+-auto-deploy-[0-9]+$/ || $CI_COMMIT_TAG =~ /^.*ee\.\d.*$/ || $CI_COMMIT_TAG =~ /^\d+\.\d+\.\d+\+[^ ]{7,}\.[^ ]{7,}$/'

triggered-ee-job:
  rules:
    - if: '%{EE_BUILD} && $CI_PIPELINE_SOURCE == 'trigger'
      when: always
    - when: never
  script:
    - bash trigger.sh

scheduled-ee-job:
  rules:
    - if: '%{EE_BUILD} && $CI_PIPELINE_SOURCE == 'schedule'
      when: always
    - when: never
  script:
    - bash schedule.sh

Here, the macro EE_BUILD is used to store the result of an expression (is this an EE build). Then, that macro is reused in if directives instead of that long expression.