feat: federation authority test plans

[peppelinux]

This PR resolves #770

[peppelinux]

@fmarino-ipzs @giadas @Sh-Amir could we re-use the existing test plans already used in currennt federation impl?

[Copilot]

Pull Request Overview

This PR adds comprehensive test plans for Federation Authorities (Trust Anchors and Intermediates) to the documentation, addressing issue #770. The new test plans define validation and conformance criteria for the Trust Infrastructure.

Key Changes:

• Adds a new test plan document covering 20 test cases for Federation Authority functionality
• Includes validation for Entity Configuration, Subordinate Statements, and federation registry endpoints
• Provides both Italian and English versions of the documentation

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
docs/it/test-plans.rst	Adds reference to the new Italian Federation Authority test plans document
docs/it/test-plans-federation-authority.rst	New Italian documentation defining 20 test cases for Federation Authority validation
docs/en/test-plans.rst	Adds reference to the new English Federation Authority test plans document
docs/en/test-plans-federation-authority.rst	New English documentation defining 20 test cases for Federation Authority validation

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