Fix Importing Az in Parallel

[isra-fel]

Description

Fix #18321

This PR resolves the file IO conflict when importing Az.Accounts in parallel PowerShell processes.

Root cause:
Config manager is initialized during importing Az.Accounts. It tries to create the config file if it does not exist. It also tries to write that file if there are old configs need to migrate. At the meantime, it reads the file to build config values.
All these file IO operations may conflict with each other. Causing Az.Accounts fail to be imported.

Fix:

1. Add a mutex to protect the initialization of the configs. Other processes will need to wait until the previous one finishes initializing.
2. Add a safe mode that we can fall back to when configs fail to initialize. In safe mode, it does not resolve configs from file system / environment variables. There are only default values.

Design:
This PR introduced a sibling to ConfigManager as SafeConfigManager, which do not load configs from file system nor environment variables, so it's safe from unexpected exceptions.
After that the name ConfigManager seemed inappropriate so I renamed it into DefaultConfigManager and introduced an abstract base class ConfigManager for both to inherit from.
Now AzureSessionInitializer will first try to create a DefaultConfigManager and if it fails, use SafeConfigManager instead.

P.S. we might want to add this to smoke test.

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

[isra-fel]

lock is no longer needed because the initialization is guarded by a mutex

[jose-bueno-microsoft]

@isra-fel is there an ETA on this release? I have a few customers asking on this fix release. Many thanks!

[isra-fel]

Hi @jose-bueno-microsoft we are getting this reviewed and tested by different teams. Currently the ETA would be July 5th.
Update 6/20: The engineering build has been verified by Functions team.

[BethanyZhou]

Why previous code didn't work? var dataStore = settings?.DataStore ?? new MockDataStore();