Allow additional entry points in FMU code, specify restrictions

[pmai]

As per the discussion on #1632, in order to properly support both minor releases with new optional features, as well as layered standards, we should clarify in the standard that additional entry points in FMUs (binary/source code) are allowed (they already have to be to follow OS ABI requirements), and that we reserve all symbols starting with fmi3 for FMI 3.x.

[pmai]

Note that we currently require symbols for optional features to be present, even if the feature is not supported. For additional features in minor releases we cannot continue requiring this; so this might be considered a variance...