Add Admin API authorization capability to Bruno collection

[njlie]

Context

With the Admin API secured in https://github.com/interledger/rafiki/pull/2466, the Bruno collection should ideally be able to support getting the Hydra auth token and include it in requests to the Admin API.

Todos

• [ ] Add api token variable to Bruno collection that can store auth token and gets used in each request
• [ ] Add Bruno request to initiate authorization flow for Admin API