feat(restheadspec): ✨ Add support for join aliases in filters and sorts

- Extract join aliases from custom SQL JOIN clauses.
- Validate join aliases for filtering and sorting operations.
- Update documentation to reflect new functionality.
- Enhance tests for alias extraction and usage.

pkg/restheadspec/HEADERS.md

@@ -233,6 +233,27 @@ x-custom-sql-join: LEFT JOIN departments d ON d.id = e.dept_id | INNER JOIN role
 - Multiple JOINs can be specified using the pipe `|` separator
 - JOINs are sanitized for security
 - Can be specified via headers or query parameters
+- **Table aliases are automatically extracted and allowed for filtering and sorting**
+
+**Using Join Aliases in Filters and Sorts:**
+
+When you specify a custom SQL join with an alias, you can use that alias in your filter and sort parameters:
+
+```
+# Join with alias
+x-custom-sql-join: LEFT JOIN departments d ON d.id = employees.department_id
+
+# Sort by joined table column
+x-sort: d.name,employees.id
+
+# Filter by joined table column
+x-searchop-eq-d.name: Engineering
+```
+
+The system automatically:
+1. Extracts the alias from the JOIN clause (e.g., `d` from `departments d`)
+2. Validates that prefixed columns (like `d.name`) refer to valid join aliases
+3. Allows these prefixed columns in filters and sorts
 
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