Add support for client credentials OIDC flow

[Andy-Grigg]

📝 Description of the feature

We currently support the PKCE flow for OIDC, which works well for connecting to Granta MI. However, this package can/should be used for connecting to other web APIs. A commonly used flow is client credentials. We should be able to pass in a client ID and client secret, and receive back a token which we can use for authentication.

💡 Steps for implementing the feature

Issue #129 and PR #139 include implementing requests-auth which provides the client credentials flow out of the box. Assuming that PR is merged, it should just be a case of using the appropriate class and plumbing through to the ApiClient builder.

🔗 Useful links and references

https://colin-b.github.io/requests_auth/#client-credentials-flow