Add rules to input and add some validation

What does this MR do and why?

Part 1 &18546 - #563375

Turn on the feature flag ci_dynamic_pipeline_inputs

Valid syntax:

spec:
  inputs:
    deployment_type:
      options: ["rolling", "blue_green", "canary"]
      default: "rolling"

    target_environment:
      options: ["development", "staging", "production"]
      default: "development"

    resource_tier:
      rules:
        - if: $[[ inputs.target_environment ]] == "development"
          options: ["small", "medium"]
          default: "small"
        - if: $[[ inputs.target_environment ]] == "staging"
          options: ["medium", "large"]
          default: "medium"
        - if: $[[ inputs.target_environment ]] == "production" && $[[ inputs.deployment_type ]] == "canary"
          options: ["large", "xlarge", "xxlarge"]
          default: "large"
        - if: $[[ inputs.target_environment ]] == "production"
          options: ["large", "xlarge"]
          default: "xlarge"
        - options: ["small"]

---
deploy:
  script: |
    echo "Deploying using $[[ inputs.deployment_type ]] strategy"
    echo "Target: $[[ inputs.target_environment ]]"
    echo "Resources: $[[ inputs.resource_tier ]]"

Valid syntax with multiple rule conditions:

spec:
  inputs:
    cloud_provider:
      options: ["aws", "gcp", "azure"]
      default: "aws"

    environment:
      options: ["dev", "staging", "prod"]
      default: "dev"

    instance_type:
      rules:
        - if: $[[ inputs.cloud_provider ]]  == "aws" && $[[ inputs.environment ]]  == "dev"
          options: ["t3.micro", "t3.small"]
          default: "t3.micro"
        - if: $[[ inputs.cloud_provider ]] == "aws" && $[[ inputs.environment ]] == "prod"
          options: ["m5.large", "m5.xlarge"]
          default: "m5.large"
        - if: $[[ inputs.cloud_provider ]] == "gcp"
          options: ["e2-standard-2", "e2-standard-4"]
          default: "e2-standard-2"
        - options: ["standard"]

---
provision:
  script: echo "Provisioning $[[ inputs.instance_type ]] on $[[ inputs.cloud_provider ]]"

mixed syntax:

spec:
  inputs:
    region:
      options: ["us-east-1", "us-west-2", "eu-west-1"]
      default: "us-east-1"
    feature_flags:
      rules:
        - if: $[[ inputs.region ]] == "us-east-1"
          options: ["feature_a", "feature_b", "feature_c"]
        - if: $[[ inputs.region ]] == "eu-west-1"
          options: ["feature_a", "feature_d"]
          default: "feature_a"
        - options: ["feature_a"]
          default: "feature_a"

---
jobby_job:
  script: echo "$[[ inputs.region ]]"

Invalid syntax:

spec:
  inputs:
    bad_rules:
      rules:
        - if: $[[ inputs.environment ]] == "prod"
          invalid_key: "should not be allowed"
          options: ["value"]
---
test-job:
  script: echo "This should fail validation"

error: This GitLab CI configuration is invalid: header:spec:inputs:bad_rules config rule contains invalid keys: invalid_key.

NOTE: Circular dependencies will be implemented in the next issue for ease of review. We can't do it here because the editor only runs the schema validation and basic parsing, and the circular deps will only be detected when the pipeline actually gets processed.

Issue	Status	Description
Keyword support	👈	Add Input class and basic input configuration support
Input Lexme	In Review, %18.5	Adds Input lexeme to parse inputs syntax in expressions
Semantic Validation	Planned for %18.6	Statement#input_names, validate referenced inputs exist, circular dependency detection
GraphQL API	Planned	Expose fields in GraphQL, return structured data for MVP

NOTE: I am creating a spike so we can test this all out in here:

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