Add API key and legacy authentication alongside OIDC

[lennartvava]

Summary

Add API key authentication and restore the v3 configuration path alongside OIDC.

• Add ApiKeyConfiguration to authenticate using apiKey and applicationId headers.
• Add LegacyConfiguration for clients that manage their own authentication.
• Introduce a shared configuration interface and require a token cache only for OIDC.
• Preserve existing OIDC configuration signatures, protected property types, and token-provider behavior.
• Document the upgrade path from v3 to v4.3.

Validation

• 35 tests passed with 105 assertions, including OIDC subclass compatibility, cached Bearer authentication, API key headers, and legacy header preservation.
• Coding standards, static analysis, mess detection, and duplicate-code checks passed.
• Verified Composer dependency resolution with the local checkout.

Targets release 4.3.0. Existing v4 OIDC consumers require no code changes.

[github-actions]

Coverage report for commit: 780b82a
File: build/logs/clover.xml

Summary - Lines: 86.16% | Methods: 93.75%

Files	Lines	Methods	Branches
src/Client/Token
OidcTokenProvider.php	-	-	100.00%

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