[Feature Request] Region auto enable on env variable

[bgavrilMS]

Region auto-enable

1. On creation of a ConfidentialClientApplication, MSAL shall detect env variable MSAL_FORCE_REGION, which will be set to a specific region (e.g. westus1)
2. If this env variable is set, MSAL shall opt-in to ESTS-R with the value of this variable.

Disabling the behavior

If region is explicitly set in MSAL to the string "DisableMsalForceRegion`, then the above no longer applies

Conflicts with WithAzureRegion(xyz)

Use of the api WithRegion(xyz) takes precedence over the env variable.

Acceptance tests

For all of the tests, assume env variable MSAL_FORCE_REGION=eastus

1. No other config is used. ACTUAL region used: eastus
2. App developer configures region "westus" in MSAL. ACTUAL region used: westus
3. App developer configures region "DisableMsalForceRegion" in MSAL. ACTUAL region used: none

Original issue:

AzureAD/microsoft-authentication-library-for-dotnet#4930