feat(restheadspec): ✨ Add custom SQL JOIN support

- Introduced `x-custom-sql-join` header for custom SQL JOIN clauses.
- Supports single and multiple JOINs, separated by `|`.
- Enhanced query handling to apply custom JOINs directly.
- Updated documentation to reflect new functionality.
- Added tests for parsing custom SQL JOINs from query parameters and headers.

pkg/restheadspec/restheadspec.go

@@ -32,6 +32,7 @@
 //   - X-Clean-JSON: Boolean to remove null/empty fields
 //   - X-Custom-SQL-Where: Custom SQL WHERE clause (AND)
 //   - X-Custom-SQL-Or: Custom SQL WHERE clause (OR)
+//   - X-Custom-SQL-Join: Custom SQL JOIN clauses (pipe-separated for multiple)
 //
 // # Usage Example
 //