Saithriftv2 check null sai apis

[chrispsommers]

Fixes #1535. I modified the code-generator template to check for null SAI function pointers before calling, and throwing an exception with helpful message on the console.

The changed file of interest is actually just meta/templates/sai_rpc_server_functions.tt. There are more files in the change list because this PR includes changes from unmerged #1514 and #1533

Example console trace when calling switch_api->get_switch_attribute() which isn't implemented and has a null pointer. The process does not crash anymore.

Starting SAI RPC server on port 9092
NULL ptr: switch_api->get_switch_attribute

To understand how this works, the template inserts the following code (as an example) automatically:

    if (switch_api->get_switch_stats == (void *)0) {
        std::cerr << "NULL ptr: switch_api->get_switch_stats" << std::endl;
    
      sai_thrift_exception e;
      e.status = SAI_STATUS_NOT_IMPLEMENTED;
      throw e;
    }

right before making any API call such as:

    status = switch_api->get_switch_stats(switch_id, number_of_counters, sai_counter_ids, sai_counters);

[richardyu-ms]

please resolve the merge conflict

[chrispsommers]

Superceded by #1558, this PR had rebasing artifacts and was not clean.