feat: Add decompress flag to sourcemaps and files upload

[kamilogorek]

Passing --decompress to either sourcemaps upload or files upload will now detect and recompress gzip files and exact their contents before uploading.

As a nice side-effect, we don't get cannot find source-map reference errors from symbolic, as it correctly matches them too.

In addition to this change, few minor cleanups:

• remove FileContents enum and use &[u8] directly, to skip a layer of indirection
• make upload_release_file accept UploadContext to reduce number of arguments passed to this function
• remove upload_release_files which was not used

This feature will allow getsentry/sentry-webpack-plugin#195 to be resolved.