refactor: rework aged authentication config interface

[comtalyst]

Fixes #

Description Part of #224 This involves refactoring of auth config, removing some redundant code (e.g., in favor of DefaultAzureCredentials, remove some unused variables), and renaming ARM_USER_ASSIGNED_IDENTITY_ID to KUBELET_IDENTITY_CLIENT_ID for accuracy to its purpose/use. See core motivations in the issue.

How was this change tested?

• Unit tests
• E2E - Utilization (through make az-e2etests)
• E2E for NAP (using managed identity)

Does this change impact docs?

• [ ] Yes, PR includes docs updates
• [ ] Yes, issue opened: #
• [X] No

Release Note

- Deprecate `ARM_USE_CREDENTIAL_FROM_ENVIRONMENT` and `ARM_USE_MANAGED_IDENTITY_EXTENSION` in favor of track2-based authentication: https://github.com/Azure/azure-sdk-for-go/blob/main/sdk/azidentity/README.md#defaultazurecredential
- Rename `ARM_USER_ASSIGNED_IDENTITY_ID` to `KUBELET_IDENTITY_CLIENT_ID` for accuracy to its purpose/use
- Remove unused environment variables `AZURE_CLUSTER_NAME` and `ARM_VM_TYPE`

[coveralls]

Pull Request Test Coverage Report for Build 11064404930

Details

• 36 of 43 (83.72%) changed or added relevant lines in 7 files are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 97.899%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/providers/instance/azure_client.go	0	2	0.0%
pkg/providers/instance/skuclient/skuclient.go	0	5	0.0%
<!--	Total:	36	43	83.72%	-->

Files with Coverage Reduction	New Missed Lines	%
pkg/auth/util.go	2	0.0%
pkg/auth/config.go	4	73.58%
<!--	Total:	6		-->

Totals
Change from base Build 11063867966:	0.2%
Covered Lines:	36249
Relevant Lines:	37027

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💛 - Coveralls