Adds specs for the OIDC Client class

[jvanderhoof]

Implemented Changes

Adds tests (using VCR to record OIDC communication) for the OIDC Client class.

Definition of Done

At least 1 todo must be completed in the sections below for the PR to be merged.

Changelog

• [ ] The CHANGELOG has been updated, or
• [x] 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

• [ ] Docs (e.g. READMEs) were updated in this PR
• [ ] A follow-up issue to update official docs has been filed here: insert issue ID
• [x] This PR does not require updating any documentation

Behavior

• [ ] This PR changes product behavior and has been reviewed by a PO, or
• [ ] These changes are part of a larger initiative that will be reviewed later, or
• [ ] No behavior was changed with this PR

Security

• [ ] Security architect has reviewed the changes in this PR,
• [ ] These changes are part of a larger initiative with a separate security review, or
• [x] There are no security aspects to these changes