Add function request_sobject_by_external_id to ExForce

[hmaiabrex]

I noticed that ExForce does not have a function to request_sobject_by_external_id. We do have the get_sobject_by_external_id, but it only implements the "GET" method for the REST API endpoint.

I'll make a PR adding this function, so that we can request sObjects by external id. This will enable upserting sObjects by external id, for example.

• sObject Rows by External ID
• Insert or Update (Upsert) a Record Using an External ID