7056: AzureConnection - if preferred cloud shell wasn't set let the user choose rather than crashing

[Don-Vito]

Summary of the Pull Request

This is an alternative implementation for #8197.
In rare occurrences, the shell settings might exist, but have no preferred shell configured.
Usually this will happen if the setting were created with old version of the cloud shell API.
Currently, we crash in such scenarios while trying to read a non-existing value.
This PR allows handles such cases by letting the user choose between PowerShell and Bash.

References

See #8197 for alternative solution (that simply fails the connection with informative message).

PR Checklist

• Closes Azure Cloud Shell keeps resulting in "Key not found" error #7056
• CLA signed.
• Tests added/passed
• Documentation updated.
• Schema updated. - Irrelevant
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

• Manual testing
• Most of the testing was done with windbg changing the flow as if the preferred shell does not exist, as the problem does not reproduce in my subscriptions anymore

[DHowett]

This is nuanced, but for places where we know we're in-bounds we can use til::at. gsl::at does a bounds check 😄 which we don't need here.

[Don-Vito]

Thanks! :) I was looking for it 😊 Fixing it.

[zadjii-msft]

Hey this one's no longer relevant with #8197 having merged, right?