Streams are read until the end even if it's not necessary

[fl-ido]

Hello,

The DataFilePrivate constructor always read until the end of the given stream in order to get the length of the data: https://github.com/open-eid/libdigidocpp/blob/efb30bc82d844f98fdd74102d9b63789e4a86bd4/src/DataFile.cpp#L93C1-L95C47

It is not a problem for streams coming from files on disk, as we can access them several times. However, in case of a HTTP socket for instance, we only have the data once; so, reading until the end of the stream means bufferizing the whole data in memory, which is not what we want when using streams.

I see that this size is not used for central purposes in the code, but only for tracing/testing/examples purposes. Would it be possible to move this m_size initialization to the fileSize function implementation (https://github.com/open-eid/libdigidocpp/blob/efb30bc82d844f98fdd74102d9b63789e4a86bd4/src/DataFile_p.h#L38C2-L38C57) ? By doing this, the stream would not be read by default, but only when the caller really wants to know the size.