Simplify TarStreamBuilder#addEntry() to deal with bytes

[briandealwis]

From #627 (review)

TarStreamBuilder#addEntry(String content, String fileLocation) seems to be called by ImageToTarballTranslator#toTarballBlob() to add container configuration and manifest files. Oddly the content of these files is passed as a String such that the TarStreamBuilder is responsible for performing an text to byte encoding. Rather than provide two variants of TarStreamBuilder#addEntry(), why don't we just pass the file contents as byte[] and let toTarballBlob() deal with the encoding?

[elharo]

Agreed. Something looks very off here. Output streams should work on bytes, not strings. Passing file content as a string is a huge red flag.

[coollog]

I think the original point of using String as a parameter type is to prevent callers from accidentally using the addEntry(String content, String fileLocation) signature for files (as opposed to addEntry(TarArchiveEntry entry)), where they accidentally read the file into a byte[] and then call this method. For files, the addEntry(TarArchiveEntry entry) signature should be used so that files are never read into memory.