Finish support for `cert_file` parameter

[sgnn7]

While support for this parameter was partially in the codebase, the documentation was lacking and we didn't have good unit tests around this. This change ensures that we have this capability tested and documented just as much as the other features.

What ticket does this PR close?

Connected to #105

Checklists

Change log

• [x] The CHANGELOG has been updated, or
• [ ] This PR does not include user-facing changes and doesn't require a CHANGELOG update

Test coverage

• [x] This PR includes new unit and integration tests to go with the code changes, or
• [ ] The changes in this PR do not require tests

Documentation

• [x] Docs (e.g. READMEs) were updated in this PR, and/or there is a follow-on issue to update docs, or
• [ ] This PR does not require updating any documentation

[sgnn7]

Blocked until v3.0.0 gets released