feat(codepipeline): pipeline type v2 with pipeline-level variables an…

…d triggers (aws#28538)

This PR supports pipeline type v2 with pipeline-level variables and triggers.

When referring to a variable in pipeline actions, it must be specified according to the format `#{variables.variableName}`. In order to avoid the need to specify directly in this form, a new class `Variable` with a `reference()` method was created.

```ts
const myVariable = new codepipeline.Variable({
  variableName: 'bucket-var',
  description: 'description',
  defaultValue: 'sample',
});

const pipeline = new codepipeline.Pipeline(stack, 'Pipeline', {
  artifactBucket: sourceBucket,
  pipelineType: codepipeline.PipelineType.V2,
  variables: [myVariable],
  stages: [
    {
      stageName: 'Source',
      actions: [sourceAction],
    },
    {
      stageName: 'Deploy',
      actions: [
        new S3DeployAction({
          actionName: 'DeployAction',
          extract: false,
          // objectKey: '#{variables.bucket-var}.txt',
          objectKey: `${myVariable.reference()}.txt`,
          input: sourceOutput,
          bucket: deployBucket,
        }),
      ],
    },
  ],
});
```

- user guide
  - https://docs.aws.amazon.com/codepipeline/latest/userguide/pipeline-types-planning.html
  - https://docs.aws.amazon.com/codepipeline/latest/userguide/reference-variables.html#reference-variables-workflow
  - https://docs.aws.amazon.com/codepipeline/latest/userguide/tutorials-github-tags.html
- CloudFormation
  - https://docs.aws.amazon.com/ja_jp/AWSCloudFormation/latest/UserGuide/aws-resource-codepipeline-pipeline.html
  - https://docs.aws.amazon.com/ja_jp/AWSCloudFormation/latest/UserGuide/aws-properties-codepipeline-pipeline-variabledeclaration.html
  - https://docs.aws.amazon.com/ja_jp/AWSCloudFormation/latest/UserGuide/aws-properties-codepipeline-pipeline-pipelinetriggerdeclaration.html

Closes aws#28476 aws#28694.

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*