feat: add auth package

[matt-FFFFFF]

Fixes Azure/Azure-Landing-Zones#1492

This pull request introduces a new internal authentication helper package, refactors all authentication logic to use this new helper, and adds tests and documentation for it. Additionally, it updates the Go version, modifies CI to support 32-bit builds, and adjusts build targets.

Authentication Refactor and Improvements

• Introduced a new internal package internal/auth with a helper function NewToken() to create Azure credentials using well-known Azure/Terraform environment variables and conventions. This centralizes and simplifies authentication logic for the entire codebase. [1] [2]
• Replaced all direct usage of azidentity.NewDefaultAzureCredential with the new auth.NewToken() helper throughout the codebase and tests, ensuring consistent and maintainable authentication handling. [1] [2] [3] [4] [5] [6] [7] [8] [9]

Testing and Documentation

• Added a comprehensive test suite for the new authentication helper (internal/auth/auth_test.go) to ensure correct environment variable parsing and logic.
• Added package documentation (internal/auth/doc.go) explaining usage, supported environment variables, and behavior of the new authentication helper.

Build and CI Enhancements

• Updated the Go version in go.mod to 1.25.0 and added a dependency on github.com/Azure/entrauth.
• Added a 32-bit Linux build job to CI (.github/workflows/go-test.yml) and excluded unsupported build targets in .goreleaser.yml. [1] [2]

Imports and Code Cleanup

• Updated import statements across the codebase to use the new internal/auth package instead of importing azidentity directly. [1] [2] [3] [4] [5] [6]

These changes collectively improve maintainability, testability, and CI support for the project.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 23.37662% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.07%. Comparing base (bf9ce45) to head (fed9978).

Files with missing lines	Patch %	Lines
internal/auth/auth.go	22.66%	58 Missing ⚠️
cmd/alzlibtool/command/check/library.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     Azure/alzlib#219      +/-   ##
==========================================
- Coverage   45.46%   45.07%   -0.39%     
==========================================
  Files          48       49       +1     
  Lines        4342     4417      +75     
==========================================
+ Hits         1974     1991      +17     
- Misses       2129     2187      +58     
  Partials      239      239              

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