Added 3 configs to config framework

[isra-fel]

Description

Includes the definition of the following configs:

• DefaultSubscriptionForLogin
• DisplayBreakingChangeWarnings
• EnableDataCollection
  • Including how to migrate old settings to config framework
• EnableSurvey (delayed)

And improvements to config framework and cmdlets:

• Removed "-All" from Clear-AzConfig
• Introduced src/shared/ConfigKeys.cs which is included in every service project so they can share the same definition for the keys.
• Combined environment variable configs from different targets (machine, user, process) together because it's not distinguishable if an environment variable is set on which target.
  • By doing this, the priority of config providers becomes: Config (Process) > Environment Variable > Config (Current User)

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
    • {Please put the link here}
  • the markdown help files have been regenerated using the commands listed here