Investigate and reduce usage of com.nimbusds dependencies

[Avery-Dunn]

We use multiple com.nimbusds.* packages throughout the library to handle standard OAauth/JWT/JSON/etc. behavior.

However, over the years it's become clear that usage of those packages have led to some bad practices, such as unnecessary parsing, poor exception handling, and just a general lack of clarity about the code flow.

Our usage of those packages should be audited, and wherever possible we should limit our reliance on them.