Parse Expressions & Rules for inputs

Context

Part 2/8: Dynamic Input Selection in GitLab Pipelines pla... (#520094 - closed) - this will parse and evaluate expressions.

Build the core expression evaluation engine that powers dynamic input rules. This engine parses and evaluates conditional expressions from the rules.

# Simple 
- if: $[[ inputs.cloud_provider ]] == 'aws'
  options: ['t3.micro', 't3.small']

# Complex 
- if: $[[ inputs.cloud_provider ]] == 'aws' && $[[ inputs.environment ]] == 'development'
  options: ['t3.micro', 't3.small']
  default: 't3.micro'

Notes

• Schema will be extendible for additional operators post-MVP - != and || withouth breaking changes

Error Handling:

• Invalid syntax: clear parse error messages
• Missing inputs: graceful handling with boolean result
• Unsupported operators: helpful error messages

This will be split up into two MRs:

MR 1 - Expression infrastructure & parsing !206891

MR 2 - More input validation and circular deps