Port WorkloadIdentityCredential's auth flow to ClientAssertionCredential.

[anannya03]

Description

Porting WorkloadIdentityCredential's auth flow to ClientAssertionCredential.

Porting Auth Flow

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[Copilot]

Pull Request Overview

This PR refactors the WorkloadIdentityCredential class to use ClientAssertionCredential internally instead of the lower-level IdentityClient and IdentitySyncClient implementations. The change simplifies the implementation by delegating to an existing credential type while maintaining the same external API.

• Replaces direct use of IdentityClient/IdentitySyncClient with ClientAssertionCredential
• Introduces a private method to read federated tokens from files
• Updates constructor to build ClientAssertionCredential with proper configuration transfer

Comments suppressed due to low confidence (1)

sdk/identity/azure-identity/src/main/java/com/azure/identity/WorkloadIdentityCredential.java:148

• The new readFederatedTokenFromFile method lacks test coverage. This method handles file I/O and exception scenarios that should be thoroughly tested, including IOException handling and file reading edge cases.

    private String readFederatedTokenFromFile(String filePath) {