Add Auth Options: User-Managed Identity, System-Managed Identity, and AzDO Workload Identity Federation

[ianjensenisme]

This pull request introduces several new authentication methods and updates the documentation accordingly. The main changes include adding support for Managed Identity and Azure DevOps Workload Identity Federation authentication, along with corresponding updates to the documentation and tests.

Authentication Methods Added:

• Added support for Managed Identity authentication, including both system-managed and user-managed identities (internal/api/auth.go).
• Added support for Azure DevOps Workload Identity Federation authentication (internal/api/auth.go).

Documentation Updates:

• Updated docs/index.md to include new sections for Managed Identity and Workload Identity Federation authentication methods. [1] [2] [3] [4]
• Added new environment variables related to the new authentication methods (docs/index.md).

Configuration Changes:

• Updated ProviderConfig struct to include new fields for Managed Identity and Azure DevOps Workload Identity Federation (internal/config/config.go). [1] [2]
• Added new methods to check if the new authentication methods are provided (internal/config/config.go). [1] [2]

Tests:

• Added unit tests for the new authentication methods to ensure they handle various error scenarios correctly (internal/api/client_test.go).

Changelog:

• Added an entry to the changelog for the new authentication methods (.changes/unreleased/added-20241218.yaml).This pull request introduces new authentication methods for the Power Platform provider in Terraform. The key changes include adding support for Managed Identity and Workload Identity Federation, along with the necessary configuration updates and new methods for handling these authentication mechanisms.

Documentation Updates:

• Added sections for "Authenticating to Power Platform using a Managed Identity" and "Authenticating to Power Platform using Workload Identity Federation" in docs/index.md. [1] [2] [3]

Codebase Enhancements:

• Introduced new methods in internal/api/auth.go to handle authentication using User-Managed Identity, System-Managed Identity, and Azure DevOps Workload Identity Federation. [1] [2]

Configuration Changes:

• Added UseMsi and AzDOServiceConnectionID to the ProviderConfig struct in internal/config/config.go and updated related methods to check for these configurations. [1] [2] [3] [4] [5]

Environment Variables:

• Added new environment variables POWER_PLATFORM_USE_MSI and POWER_PLATFORM_AZDO_SERVICE_CONNECTION_ID in internal/constants/constants.go. (internal/constants/constants.goR112-R117, F52f86bdL162R162)

Provider Configuration:

• Updated the provider schema and configuration logic in internal/provider/provider.go to include the new authentication options. [1] [2] [3]