Add the ability to track Users

Description of the change

This PR adds the ability to set and unset a User, which will be sent with every report to Rollbar, which in turn, will automatically add the option to organize occurrences and items on a per-User basis.

Type of change

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

Related issues

Shortcut stories and GitHub issues (delete irrelevant)

Fix https://github.com/rollbar/rollbar-flutter/issues/79

Checklists

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] "Ready for review" label attached to the PR and reviewers assigned
[x] Issue from task tracker has a link to this pull request
[ ] Changes have been reviewed by at least one other engineer

rollbar / rollbar-flutter

Add the ability to track Users #80