add selectors and azclient cleanup

[davidgamero]

Description

Changes in setup-gh command:

• Centralize Credentials management in AzClient
• Abstract AzClient out of the SetupCommand
• Provide a default app name of "-workflow"
• Use the Select Prompt for Tenant and ResourceGroup
• Autofill gh repo name default value using gh cli
• Allow overriding validators (gh repo/az cli validity checks)
• Update Select Searcher to allow matching out-of-order terms (space delimited, case-insensitive)