Refactors `codersdk.Client`'s use of session tokens to use a `SessionTokenProvider`, which abstracts the obtaining and storing of the session token.
The main motiviation is to unify Agent authentication an an upstack PR, which can use cloud instance identity via token exchange, rather than a fixed session token.
However, the abstraction could also allow functionality like obtaining the session token from other external sources like the OS credential manager, or an external secret/key management system like Vault.
Partially implements https://github.com/coder/internal/issues/893
This isn't the full implementation of `coder exp tasks create` as
defined in the issue, but it is the minimum required to create a task.