Preserve DNS provider metadata for DNS-01 cleanup

[bjornmp]

Summary

This change stores the full libdns.Record returned by AppendRecords while a DNS-01 challenge is active, instead of reducing it to record.RR() immediately.

The portable DNS fields are still accessed through record.RR() for propagation checks, logging, and memory lookup. Cleanup now passes the original returned record back to DeleteRecords, preserving provider-specific metadata such as ProviderData.

Why

Some libdns providers receive an internal record ID from the provider API when creating a record and store it in ProviderData. If CertMagic discards that metadata, cleanup may need to list zone records and rediscover the provider ID before deletion.

Preserving the returned record is more robust:

• Providers with record IDs can delete directly without an extra list call.
• Tokens do not need list permissions solely to rediscover an ID that was already returned during creation.
• Providers without metadata continue to work because the standard DNS data remains available through Record.RR().

This does not make CertMagic depend on provider-specific IDs. It only avoids discarding metadata when a provider supplies it.

Validation

Added a regression unit test verifying that cleanup receives the original returned record including ProviderData.

Tested with:

go test ./...

Fixes #381

[mholt]

Fair enough, didn't realize ProviderData was that useful, but this makes sense! Thanks.