Add default payload keys before the custom transform runs

[waltjones]

Description of the change

Currently the defaults from the config override the updated values from the user transform function. Instead, the transform should override the defaults.

This PR adds the default payload options in its own step, so that it is present during the user transform.

Type of change

• [x] Bug fix (non-breaking change that fixes an issue)

Related issues

Fixes https://app.shortcut.com/rollbar/story/120633/ https://github.com/rollbar/rollbar.js/issues/1026

Development

• [x] Lint rules pass locally
• [x] The code changed/added as part of this pull request has been covered with tests
• [x] All tests related to the changed code pass in development

Code review

• [x] This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• [x] Issue from task tracker has a link to this pull request
• [x] Changes have been reviewed by at least one other engineer