ACME: mark unused directory resources as optional.

[bavshin-f5]

We discovered a server implementation that does not support key rollover and indicates that by not sending "keyChange".
For completeness, I'm marking all the currently unused directory resources as optional.

[Copilot]

Pull Request Overview

This PR updates the ACME directory resource fields to handle server implementations that don't support all directory resources, specifically addressing servers that don't support key rollover by not sending "keyChange".

• Changes revoke_cert and key_change fields from required Uri to optional Option<Uri>
• Updates serde attributes to use optional URI deserialization with default values

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[ensh63]

One comment: unused fields may be removed from structure definition. If desired, they definitions may be commented out to simplify restoring when code will use them.