Add support for rollbar-agent

[bishopb]

Description of the change

The configuration for logging in Laravel requires an instance of Monolog\Handler\HandlerInterface, but the Rollbar library requires a string with value of "agent" to use the agent. This disconnect makes it impossible to use the Rollbar agent with Laravel.

Fixes Issue #85, by way of an adapter class. An example Laravel configuration to use the adapter:

'rollbar' => [
      'driver' => 'monolog',
      'handler' => \Rollbar\Laravel\AgentHandler::class,
      'level' => 'debug',
      'access_token' => env('ROLLBAR_TOKEN'),
      'environment' => 'production'
],

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)

Related issues

Fixes Issue #85.

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 mentioned in a comment
• [x] Changes have been reviewed by at least one other engineer
• [x] Issue from task tracker has a link to this pull request