cdk-codepipeline-slack support approval via specify user.

[neilkuan]

cdk-codepipeline-slack : Support only specific users who can approve.

Example: Check Only Slack User A can click approval bottom.

new SlackApprovalAction({
            actionName: 'SlackApproval',
            slackBotToken: 'botToken',
            slackSigningSecret: 'signingSecret',
            slackChannel: 'channel',
            slackBotName: 'botName',
            additionalInformation: 'snapshotTest',
            approvalUser: ['user-a', 'user-b'], // slack user.name 
          }),