Add possibility to distinguish VAT-opted properties

[ghost]

Problem description
Investment properties can either be opted to tax or not (VAT).
This information might be relevant for a transaction but is not available in SwissRETS.

Ist ein Objekt MWST-optiert, so unterliegt der Bruttomietzins der Mehrwertsteuer.
Dadurch kann der Vorsteuerabzug geltend gemacht werden.

Desired Solution
A way to indicate whether a property is opted to tax or not.
This information is relevant predominantly for the transaction of investment properties.

For regular rental properties that information might not be too relevant.

[jensstalder]

@rgutzwiller
I'm sorry for my ignorance within the real-estate business. What do you think is the default. opted or not opted. This depends how we implement it:
<isVatOpted>applies</isVatOpted> or <isVatExempt>applies</isVatExempt> Or what would be an appropriate characteristic? Just to make sure interpretation is correct by default aka. intuitively.

[ghost]

Default is not opted for VAT and does not need to me mentioned explicitly, if a property is opted for VAT then it needs to be mentioned.

This concerns mainly renting of garages or commercial real estate (e.g. a company rents out a garage to a private person without this person also renting a flat where they live, or if a company rents out an office/commercial property to another company or private person.