Add --socks argument for scanning via SOCKS5 proxy (#347)

* Add --socks argument for scanning via SOCKS5 proxy

Implement SOCKS5 proxy support without external dependencies:
- Add socks_proxy field to AuditConf
- Add --socks host:port CLI argument with input validation
- Implement SOCKS5 handshake (no-auth, domain-name addressing) in
  SSH_Socket._connect_via_socks5()

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Michał Majchrowicz <sectroyer@gmail.com>

* Add SOCKS proxy regression tests

---------

Signed-off-by: Michał Majchrowicz <sectroyer@gmail.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/ssh_audit/auditconf.py

@@ -67,6 +67,7 @@ class AuditConf:
         self.conn_rate_test_enabled = False
         self.conn_rate_test_threads = 0
         self.conn_rate_test_target_rate = 0
+        self.socks_proxy: Optional[str] = None  # SOCKS5 proxy in "host:port" format
 
 
     def __setattr__(self, name: str, value: Union[str, int, float, bool, Sequence[int]]) -> None:
@@ -95,7 +96,7 @@ class AuditConf:
             if value == -1.0:
                 raise ValueError('invalid timeout: {}'.format(value))
             valid = True
-        elif name in ['ip_version_preference', 'lookup', 'policy_file', 'policy', 'target_file', 'target_list', 'gex_test']:
+        elif name in ['ip_version_preference', 'lookup', 'policy_file', 'policy', 'target_file', 'target_list', 'gex_test', 'socks_proxy']:
             valid = True
         elif name == "threads":
             valid, num_threads = True, Utils.parse_int(value)