[HLAPI] Add x-singleton and x-graphql-resolver

[cconard96]

Documentation for new HLAPI features added in glpi-project/glpi#24231

[SebSept]

some problems occurs with these changes :

$make livehtml

/docs/source/devapi/hlapi/schemas.rst:197: ERROR: Error parsing content block for the "list-table" directive: uniform two-level bullet list expected, but row 17 does not contain the same number of items as row 1 (5 vs 4).
                                                                                                                                                                                                                                                                                                                                                                                                                                  
.. list-table:: Extension Properties                                                                                                                                                                                                                                                                                                                                                                                              
    :widths: 25 50 25 25                                                                                                                                                                                                                                                                                                                                                                                                          
    :header-rows: 1                                                                                                                                                                                                                                                                                                                                                                                                               
                                                                                                                                                                                                                                                                                                                                                                                                                                  
    * - Property                                                                                                                                                                                                                                                                                                                                                                                                                  
    ...