Replace salvo-acme with certon for enhanced ACME support

[chrislearn]

This pull request updates the ACME crate to leverage the certon library, introducing advanced certificate management features and refactoring the configuration system. The changes modernize the ACME configuration builder, enabling support for multiple issuers, custom solvers, flexible storage, and new challenge types. Additionally, dependencies and features are updated to reflect the transition to certon.

Integration of certon and dependency updates:

• Added certon as a dependency in Cargo.toml, and removed legacy dependencies and features related to certificate management. The default feature set is now empty, and feature flags are simplified. [1] [2]
• Updated the crate description to indicate it is now powered by certon.

Configuration system refactor:

• Refactored AcmeConfig and AcmeConfigBuilder in config.rs to support new fields and options from certon, including key type selection, multiple issuers, custom storage, advanced solvers, OCSP stapling, on-demand TLS, and ZeroSSL integration. [1] [2] [3] [4]
• Improved documentation and debug output for configuration structures, making the builder API easier to use and understand. [1] [2]

New builder methods for advanced features:

• Added builder methods for DNS-01 challenges, custom solvers, key type, issuers, storage, OCSP, on-demand TLS, ZeroSSL API key, and terms of service agreement. These methods enable flexible and extensible certificate management. [1] [2]

API and documentation improvements:

• Enhanced builder method documentation for clarity and completeness, including updated descriptions for contact emails, challenge types, cache path, and certificate renewal timing. [1] [2] [3] [4]

Removal of legacy client implementation:

• Removed the legacy ACME client implementation in client.rs, as certificate issuance and challenge handling are now managed via certon.