Optionally manage feed providers via api / client and store in redis

[testower]

Is your feature request related to a problem? Please describe.

It is cumbersome to update feeds by redeploying the application, especially when config is part of source control and has to pass through various stages of CI/CD, as is the case at Entur.

Goal / high level use-case

The goal is to be able to manage feeds via the admin-api, and that the config should be kept across restarts (e.g. in redis). This also entails that the application must respond to changes in the config by adding / removing subscriptions, cleaning up etc. I'd also like to add a very light-weight admin client to facilitate the usage of the admin api. Keeping in mind, only basic auth is supported still for the admin endpoints.

Describe the solution you'd like

Some CRUD endpoints for managing feeds / providers under the admin umbrella. Use redis for persistence. Create a light-weight client that can be at the root of /admin. It can also support other existing admin functions. All of this should be configurable.

Consider how we handle credentials, can we store them in redis - do we need to encrypt them?

Describe alternatives you've considered

Allow the config be reloaded dynamically (e.g #218 ).

[testower]

As a lightweight version of this we could also allow disabling / enabling feeds in memory with restarts

[testower]

Option to read/write config file from/to disk