adding support for source attribute variable internal use only

[prklm10]

https://browserstack.atlassian.net/browse/PER-3733
adding support for source param in build post request body.
New request body atrribute for build post request to api

attributes: {
  type: projectType,
  branch: branch,
  'target-branch': target.branch,
  'target-commit-sha': target.commit,
  'commit-sha': git.sha,
  'commit-committed-at': git.committedAt,
  'commit-author-name': git.authorName,
  'commit-author-email': git.authorEmail,
  'commit-committer-name': git.committerName,
  'commit-committer-email': git.committerEmail,
  'commit-message': git.message,
  'pull-request-number': pullRequest,
  'parallel-nonce': parallel.nonce,
  'parallel-total-shards': parallel.total,
  partial: this.env.partial,
  tags: tagsArr,
  source: source // 'user_created' or 'auto_enabled_group'
},