Export SetContext to support mocking Transport

[pawelsz-rb]

Description of the change

Export the setContext method of the Transport interface to allow mocking via reimplmentation.

This build includes automatic and indirect reference of new package of rollbar-go/errors in the go.mod and go.sum files

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

Shortcut stories and GitHub issues (delete irrelevant)

• Fix Unexported function on the Transport interface prevents mocking  #104

Checklists

Development

• Lint rules pass locally (I was unable to determine what lint rules this might be referencing)
• 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