Validate CRC32c if x-goog-hash header is present

[jonasfj]

The x-goog-hash header is present in responses from GCS, see: https://cloud.google.com/storage/docs/hashes-etags

Please note that if the object was transcoded by GCS then the has will be invalid.
Also the validate must happen before gzip decompression, if the response is gzipped, and the request must be made with Accept-Encoding: gzip.

If we get this wrong, we won't be able to gzip data in GCS in the future, let's aim to avoid that.

[jonasfj]

Probably we can use package:crclib.

[jonasfj]

This is actually in:
https://pub.dev/documentation/crclib/latest/crclib/Crc32Iscsi-class.html

After viewing http.dart with @sigurdm it seems we'll have to do a minor refactor of all http handling before we can retry errors during the response phase of the requests.

[jonasfj]

There is also a crc32 implementation here:
https://github.com/dart-lang/pana/blob/master/lib/src/license_detection/crc32.dart

Thought this isn't the variant used by GCS, but it could probably be tweaked.