Add dns-account-01 challenge support

[sheurich]

This PR adds support for the new dns-account-01 challenge type to the ACME client, enabling concurrent domain validation by different clients using account-specific DNS records.

Summary

• Add dns-account-01 challenge implementation following RFC draft-ietf-acme-dns-account-label-01
• Provide account-specific DNS record names to avoid validation conflicts
• Maintain full backward compatibility with existing challenge types

Changes

• New challenge class: DNSAccount01 with account-specific record name generation
• Authorization integration: Added dns_account_01 and dns_account helper methods
• Comprehensive documentation: Dedicated README section with usage examples
• Complete test coverage: Unit tests and VCR cassettes for the new challenge type

Benefits

• Concurrent validation: Multiple ACME clients can validate the same domain simultaneously
• No conflicts: Each client gets a unique DNS record name based on account URL
• Drop-in replacement: Can be used anywhere DNS-01 challenges are used

Usage

challenge = authorization.dns_account
challenge.record_name    # => '_ujmmovf2vn55tgye._acme-challenge'
challenge.record_type    # => 'TXT'
challenge.record_content # => 'HRV3PS5sRDyV-ous4HJk4z24s5JjmUTjcCaUjFt28-8'

Testing

• All existing tests pass
• New tests verify challenge creation, record generation, and API integration
• Added VCR cassette for dns-account-01 challenge interactions

[sheurich]

Fixes #255

[sheurich]

@unixcharles for context this code interoperates with

• letsencrypt/boulder via feat: Add core definitions for dns-account-01 letsencrypt/boulder#8140 and feat: Support for dns-account-01 Challenge letsencrypt/boulder#8149
• letsencrypt/pebble via Add "dns-account-01" support from draft-ietf-acme-scoped-dns-challenges letsencrypt/pebble#435 (updated by use newer validation subdomain for dns-account-01 (fix CI eggsampler/acme error) letsencrypt/pebble#498)

and mirrors the implementation in

• eggsampler/acme via Add dns-account-01 support from draft-ietf-acme-scoped-dns-challenges eggsampler/acme#21 (updated by fix(dns-account-01): update validation domain to remove scope eggsampler/acme#26)

[unixcharles]

Thank you for the contribution

[unixcharles]

released in 2.0.26