Update instance selection behavior

[kt474]

Summary

When no instance is specified:

QiskitRuntimeService()

Warning:
Instance was not set at service instantiation. Free and trial plan instances will be prioritized. Based on the following filters, (tags: None, region: us-east, eu-de) and available plans: (premium, internal), the available account instances are: services-internal-eu, services-premium-eu, services-internal-us, services-premium-us. If you need a specific instance set it explicitly either by using a saved account with a saved default instance or passing it in directly to QiskitRuntimeService().

Filtering by region and plans_preference:

QiskitRuntimeService(region="us-east", plans_preference=["internal"])

Warning:
Instance was not set at service instantiation. Based on the following filters, (tags: None, region: us-east, plans_preference: internal), the available account instances are: services-internal-us. If you need a specific instance set it explicitly either by using a saved account with a saved default instance or passing it in directly to QiskitRuntimeService()

if plans_preference is given, it will be added to list of filters in the warning message. If plans_preference is not given, the available plans will be listed instead.

Retrieving backends:

service.backends()
// or 
service.backend('...')

Warning:
Loading instance: services-internal-eu, plan: internal

Using saved account vs. new account

If using saved account:

Loading default saved account or Loading saved account: quantum-internal

If token is passed in:

Loading account with the given token. A saved account will not be used.

Details and comments

Fixes #2374
Fixes #2346

[kt474]

@ElePT Now that the warning message wording is more or less finalized, I think this is ready for a review

[ElePT]

Account intialization by token looks good. Let's have an offline discussion regarding instance selection filters before moving forward with the other changes.

[kt474]

Updated the PR description so all warning messages are up to date. The behavior has changed that so when no instance is given and plans_preference is not set, the available instances will be prioritized by instance pricing type: free, trial, paygo, paid, subscription.

[ElePT]

Thanks a lot!! I think that this PR is in a really good state, I just have a couple of minor grammatical suggestions. I played around with different account saving/loading strategies and things seem to work as expected, and the warnings are very informative.

[ElePT]

LGTM