[Feature] Add SPF flattening tool with macro preservation

[Feature] Add SPF flattening tool with macro preservation

- Add new 'spf-flatten' command to dns_tool for optimizing SPF records
- Introduce RSPAMD_SPF_FLAG_MACRO_UNRESOLVED flag to preserve SPF macros
- Prevent macro expansion when sender IP is unavailable (flatten mode)
- SPF elements with macros (exists:, a:, mx:, ptr:) now preserved correctly
- Add multiple output formats: default, json, compact (BIND-style)
- Optimize IP addresses by removing default /32 and /128 masks
- Automatically split large SPF records into multiple includes
- Preserve qualifiers and 'all' mechanism in flattened records