Add back configurable access token creation plugin support

**Is your feature request related to a problem? Please describe.**
IMS access token creation currently only supports JWT based, OAuth2, and CLI access tokens. Other access token types might be added in the future. So it would be beneficial to add back configurable token creation plugin support to add more types of access token creation.

**Describe the solution you'd like**
* Expose API in the `Context` class to retrieve and update additional token creation plugins
* Add support to the token generation function to consider the configured token creation plugins in the selection process

**Describe alternatives you've considered**
n/a

**Additional context**
n/a

Start by locating the Context class and the token generation function, then review how JWT, OAuth2, and CLI access tokens are currently selected. Done means Context can retrieve and update additional token creation plugins, and token generation considers those configured plugins when selecting a token type.

Written by the indexing model from the issue text.