Description of the change

(duplicate of #91)
This is to help achieve parody with the PHP SDK which allows for
additional fields to be specified on the Person object. We use this to
provide details like what account a specific user belongs to or other
additional internal metadata that is helpful when debugging user/account
level errors.

This is mainly mirroring what I saw in the PHP SDK here:

https://github.com/rollbar/rollbar-php/blob/89c9069e8532acb638f2fb9efd36b529ec25354f/src/Payload/Person.php#L68-L80

    public function serialize()
    {
        $result = array(
            "id" => $this->id,
            "username" => $this->username,
            "email" => $this->email,
        );
        foreach ($this->extra as $key => $val) {
            $result[$key] = $val;
        }
        
        return $this->utilities()->serializeForRollbarInternal($result, array_keys($this->extra));
    }

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Maintenance
• New release

Related issues

From what I've seen there are not any GitHub issues for this.

Checklists

Development

• Lint rules pass locally
  (I don't see any editor config or linting documentation in the README or anywhere else in the repo?)
• The code changed/added as part of this pull request has been covered with tests
  (This code should be covered by existing unit tests, but happy to add a specific test for this code path)
• All tests related to the changed code pass in development

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• "Ready for review" label attached to the PR and reviewers assigned
• Issue from task tracker has a link to this pull request
• Changes have been reviewed by at least one other engineer