Make set_xcom route in API server DRY by reusing logic of XComModel.set method

[dabla]

While working on AIP-88 I needed to patch the XComModel.set method but discovered my patches weren't taken into account. That's when I discovered that the set_xcom route in API server didn't delegate the logic to the XComModel set method but actually contains an almost identical copy of the set method, which is not DRY.

Maybe there is a reason for this? I created this PR which delegate the set of an xcom to the XComModel.set and catches the ValueError's thrown by the XComModel.set to return corresponding HTTPException's like it was originally implemented. Advantage of this is that the code isn't duplicated anymore and if a fix is done in XComModel.set method, the fix is also automatically applied in the API server. There will be probably other methods where this logic is duplicated, so maybe we have to do a more in-depth review for this unless there is a specific reason why the code is duplicate which I'm not aware off.

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