Fixes #76690

Addresses the issue reported in dotnet/sdk-container-builds#192 (comment) by @rainersigwald

Before this change, it wasn't possible to write into an archive a file entry that contained an unseekable data stream. The undelying cause was that we were calling Length to try to get the total number of bytes of the data stream, but it is unsupported when the stream's CanSeek is false.

The way I fixed this was by separating the code that writes an entry into two possible paths:

• One path for entries with seekable data streams, which remains mostly unmodified.

• A newnew path for entries with unseekable streams, which changes the order in which we collect the entry information into the buffer:

  • First, write into the buffer all the metadata except for the size field.
  • Then, write the data stream into the archive (this is possible because we know the exact location where the data should start), counting the number of bytes that are written.
  • Afterwards, write that collected length value in the location for the size field in the buffer.
  • Finally, calculate the checksum, then write the buffer into the archive.