Fix DKIM relaxed body canonicalization and optimize performance

Fix DKIM relaxed body canonicalization and optimize performance

This PR addresses critical issues in DKIM relaxed body canonicalization and modernizes the codebase by replacing GLib types with standard C types.

- **RFC Compliance**: Fixed incorrect canonicalization of lines containing only whitespace. Previously, such lines were not properly handled according to RFC 6376, which could lead to DKIM signature verification failures.
- **Memory Safety**: Fixed incorrect pointer dereference in `rspamd_dkim_skip_empty_lines` that could cause undefined behavior.

- **Zero-copy Optimization**: Reimplemented `rspamd_dkim_relaxed_body_step` to avoid unnecessary memory copies. The new implementation:
  - Processes input data directly without intermediate buffers
  - Reduces the number of `EVP_DigestUpdate` calls by processing larger chunks
  - Improves CPU cache efficiency
  - Results in significantly better performance for large email bodies

- Replaced all GLib types with standard C equivalents:
  - `gsize` → `size_t`
  - `gssize` → `ssize_t`
  - `gboolean` → `bool`
  - `TRUE/FALSE` → `true/false`
  - And other GLib-specific types
- Added necessary standard headers (`stdbool.h`, `stdint.h`, `limits.h`)

- Added comprehensive debug logging for:
  - Chunk processing with size information
  - Empty line detection and skipping
  - Space collapsing operations

Issue: #5590