[FEATURE REQ] Adding AzureClouds and env var AZURE_CLOUD support in azure-core

[xiangyan99]

Library name

azure.core

Please describe the feature.

We are proposing to improve the cloud setting experience for our SDK. You can find the proposal here: Identity improvement proposal to add default cloud settings (github.com)

As part of it, we want to add a new enum AzureClouds and a new environment variable AZURE_CLOUD. This is the proposed azure-core change for Python: azure_cloud_setting by xiangyan99 · Pull Request #30825 · Azure/azure-sdk-for-python (github.com)

[jsquire]

Will discuss with the architects and wider Core team for triage when time allows.