add support for MariaDB ED25519 plugin

[heavycrystal]

MariaDB doesn't support caching_sha2_password or sha256_password authentication plugins, and instead has a completely different client_ed25519 plugin, which uses a modified version of the ED25519 algorithm. MariaDB recommends using this over mysql_native_password.

This PR implements support for client_ed25519. The bulk of this PR is taken from go-sql-driver/mysql#1518 with the author's permission.

Co-authored-by: @Gusted

[Copilot]

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mysql/util.go:89

• The new function 'CalcEd25519Password' should be covered by tests to ensure its correctness.

func CalcEd25519Password(scramble []byte, password string) ([]byte, error) {

[dveeden]

@heavycrystal @lance6716 could we create some unit tests for this and maybe also have a MariaDB test in CI?