Add SSH agent authentication option

[Phreak420]

Summary

Adds an SSH Authentication selector to MySQL SSH tunnel connections:

• SSH Agent (default for connections without a saved key file)
• Private Key File

Agent mode omits ssh-identity-file, allowing MySQL Shell/libssh to discover platform-agent identities, including hardware-backed YubiKeys. Private-key mode preserves the existing behavior.

Validation

• Frontend type-check passes.
• Targeted ESLint passes for changed files.
• ConnectionEditorTestConnection.spec.tsx: 3 tests pass, including agent and private-key option coverage.
• Tested end-to-end on Windows 10 with Kleopatra/Gpg4win and a YubiKey using MySQL Workbench 26.7.

Fixes #94

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