Support basic telnet functionality and add CiscoS500 device type

[srak289]

Telnet Functionality

I noticed the Telnet connection types lacked implementation and thought this could be a good starting point.

I understand telnet should not be used in production environments but for test environments where you come by a pile of cheap legacy switches that behave poorly with modern SSH kex algorithms and cipher types due to outdates IOS or otherwise .. sometimes telnet is the path of least resistance to getting your lab up and running.

Hope this is something we can continue developing, as I have several other legacy switch types I would like to support.

Cheers.

test results

(netmiko-py3.13) srak@edge:~/netmiko$ py.test tests/unit/                                                                                                                                  
/home/srak/.cache/pypoetry/virtualenvs/netmiko-3kolGDjv-py3.13/lib/python3.13/site-packages/pylama/lint/__init__.py:10: UserWarning: pkg_resources is deprecated as an API. See https://set
uptools.pypa.io/en/latest/pkg_resources.html. The pkg_resources package is slated for removal as early as 2025-11-30. Refrain from using this package or pin to Setuptools<81.             
  from pkg_resources import iter_entry_points                                                                                                                                              
=================================================================================== test session starts ===================================================================================
platform linux -- Python 3.13.5, pytest-8.3.3, pluggy-1.6.0                                                                                                                                
rootdir: /home/srak/netmiko                                                                                                                                                                
configfile: setup.cfg                                                                                                                                                                      
plugins: pylama-8.4.1, cov-4.1.0                                                                                                                                                           
collected 89 items                                                                                                                                                                         
                                                                                                                                                                                           
tests/unit/test_base_connection.py ................................                                                                                                                 [ 35%] 
tests/unit/test_clitools_helpers.py ..........                                                                                                                                      [ 47%] 
tests/unit/test_clitools_outputters.py ...                                                                                                                                          [ 50%] 
tests/unit/test_connection.py ....                                                                                                                                                  [ 55%] 
tests/unit/test_encryption_hanlding.py ....                                                                                                                                         [ 59%] 
tests/unit/test_entry_points.py .                                                                                                                                                   [ 60%] 
tests/unit/test_ssh_autodetect.py .                                                                                                                                                 [ 61%] 
tests/unit/test_utilities.py ..................................                                                                                                                     [100%]
                                                                                                                                                                                           
=================================================================================== 89 passed in 7.52s ====================================================================================

[ktbyers]

You should remove this. This is not needed.