Extract CI related logic from Git::BaseHooksService into a class

The following discussion from !184163 (merged) should be addressed:

• @fabiopitino started a discussion: (+1 comment)

  @lauraXD Can we extract a class here that is specific to how CI interprets push options? I think adding another method to Git::BaseHooksService with CI-specific internals is starting to look like we need an abstraction here.

  I've drafted below an idea which we could (in a follow-up MR) reuse in Chain::Skip#push_option_skips_ci? and even better, pass a Ci::PipelineCreation::PushOptions object to Ci::CreatePipelineService instead of a primitive hash, then stored in @command.push_options

  There are many advantages:

  1. Centralized knowledge of how CI uses push options.
  2. Less CI implementation details in Git::BaseHooksService which is already very complex.
  3. Ability to test this in isolation and with fast_spec_helper

  # lib/ci/pipeline_creation/push_options.rb
  class Ci::PipelineCreation::PushOptions
    def initialize(push_options)
      @push_options = push_options&.deep_symbolize_keys[:ci]
    end
  
    def ci_skip?
      !!push_options[:skip] # useful in Chain::Skip
    end
  
    def pipeline_variables
      raw_vars = extract_key_value_pairs_from_push_option(push_options[:variable])
  
      raw_vars.map do |key, value|
        { "key" => key, "variable_type" => "env_var", "secret_value" => value }
      end
    end
    
    def pipeline_inputs
      raw_inputs = extract_key_value_pairs_from_push_option(push_options[:input])
      ::Ci::PipelineCreation::Inputs.parse_params(raw_inputs.to_h)
    end
  
    private
  
    attr_reader :push_options
  
    def extract_key_value_pairs_from_push_option(push_option)
      result = []
      return result unless push_option
  
      push_option.each do |raw_value, _count|
        key, value = raw_value.to_s.split("=", 2)
        next if key.blank? || value.nil?
  
        result << [key, value]
      end
  
      result
    end
  end

  cc @furkanayhan