ed25519/x25519: simplify key implementation, add tests

[MichaelMure]

This also add the ed25519->x25519 conversion for private keys, notably to make sure that it matches with the conversion for public keys.

[Copilot]

Pull Request Overview

This pull request simplifies the key implementation for ed25519/x25519 conversions and adds comprehensive tests to ensure correctness and a round trip for multibase encoding. Key changes include the introduction of new tests for key generation and conversion, updates to the key conversion functions to leverage internal multibase helper functions, and corresponding updates to the ed25519 module for consistency.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
verifications/x25519/key_test.go	Adds tests for key generation, multibase round trip, and ed25519-to-x25519 conversion
verifications/x25519/key.go	Refactors key conversion logic and integrates multibase helper functions
verifications/internal/multibase.go	Introduces helper functions to encode and decode multibase public keys
verifications/ed25519/key_test.go	Adds tests for ed25519 key generation and multibase round trip
verifications/ed25519/key.go	Updates multibase functionality to use the new helper functions