Use [[nodiscard]] on some methods

[robUx4]

The result of some methods like EbmlElement::SetSizeInfinite() or IOCallback::write() should be checked.

In the case of SetSizeInfinite() the function cannot fail when setting false, so we might split the call in 2 and only return an status when setting to infinite.

@neheb pointed out that unknown attributes should be ignored, so we can safely enable it (attributes are from C+11).

BTW plenty of C++ attributes like this are making their way into C23.

[neheb]

Yeah C++ standard demands unknown attributes be ignored. Will be shown with proper warnings enabled though.