add context support

[pawelsz-rb]

Description of the change

Setting context on the rollbar Client and its async Transport

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

• Fix client/transport.Close() does not support context #80

Checklists

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• 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

[ijsnow]

Is this still supposed to be a draft?

[ijsnow]

This PR looks fine but it needs updated and unsure why it's still a draft.

[ijsnow]

I tested locally and confirmed it stopped with a cancelled context.