Make channels handling code more generic

[jyu00]

What is the expected feature or enhancement?

Today we have two channels - ibm_quantum and ibm_cloud. The URLs are hard coded, and there is channel specific logic in various places.

Since there are plans to onboard additional channels, we should make channel account management more generic. That is, to have a configuration file that lists the channels, their default urls, and any channel strategies they support.

It is unlikely the code can be completely channel agnostic, but we can try to isolate such code to separate classes/methods. If there are actions that can be done on the server side, we can also bring that back to them.

Acceptance criteria

Ideally, when adding a new channel, all one has to do it to update the config file.

Suggested fix

Instead of hard coding the channels, we should have a config file that lists what channels are support. For example, in a format of:

channels:
    - name: ibm_quantum
      default_url: api.quantum-computing.ibm.com
      token_required: True
      instance_required: True
      channel_strategy: []
    - name: ibm_cloud 
      ...

Then a new channel can just update this file with a new section. create_account can return a GenericAccount (or use the base Account since it's not actually declared as an abstract class despite the comment).

[jyu00]

This issue was brought up again in regards to new channel partners. The original PR didn't quite address the issue, as the two channels are still hard coded. For example, if I try to do

QiskitRuntimeService(channels="foo")

This would blow up as it only accepts ibm_quantum and ibm_cloud.

I updated the OP with a "Suggested fix"

[jyu00]

Closing because of pending IQP changes.