Add support for upsert by external Id

[jonathanwiesel]

I'm afraid that since there's no external id field in the standard data model I found no way to create a test for this.

Closes #269

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This change is 

[afawcett]

@jonathanwiesel in your test you can register an IDML mock impl and assert on the parameters passed to its methods. I have seen this done in another PR recently.

[jonathanwiesel]

@afawcett thank you! Will take a look to locate it and see how it goes

[jonathanwiesel]

@afawcett Instead of using the isExternalId from the DescribeFieldResult class, I changed to use the isIdLookup which is specifically for upsert operations to be more consistent and also supports the standard Id, which allowed me to create a test for it.

[jonathanwiesel]

Any feedbackcon this? Is been quite a while 💔

[ClayChipps]

Any feedbackcon this? Is been quite a while 💔

Apologies, this fell off my radar. Let me take a deep review and give this some testing. Thank you very much for the work on this. :)