Support more ACME issuers and implement a fallback logic

[tgragnato]

Feature Request

Q	A
New Feature	yes
RFC	yes
BC Break	no

Summary

If one ACME issuer experiences downtime or issues, having fallback logic enables the client to switch to an alternative issuer. This enhances the reliability of the certificate issuance process by providing a backup option in case of service disruptions with the primary issuer.

CAs may modify their APIs, policies, or service offerings over time. Supporting multiple issuers and fallback logic helps the ACME client adapt to changes in the ecosystem, ensuring ongoing compatibility and resilience.