Add support for blob rewrite

[mziccard]

This follows from #291. Main changes are:

• Add rewrite method to StorageRpc and DefaultStorageRpc
• Add BlobRewriter class to wrap multiple rewrite requests
• Add rewriter method to Storage and StorageImpl that returns a BlobRewriter
• Add unit and integration tests

[mziccard]

@aozarov I addressed all comments:

• split StorageRpc.rewrite into StorageRpc.openRewrite and StorageRpc.openRewrite
• use StorageRpc.openRewrite and StorageRpc.openRewrite to implement copy in Storage and Blob
• remane BlobRewriter to CopyWriter
• update and add tests
• update example

I also noticed that rewrite (in contrast with copy) fails if no content type is specified for the destination object.

storage.copy(CopyRequest.of("bucket", "n1", BlobInfo.builder("bucket", "n2").build()));

fails with:

{
"error": {
"errors": [
{
"domain": "global",
"reason": "required",
"message": "You must specify the content type of the destination object."
}
],
"code": 400,
"message": "You must specify the content type of the destination object."
}
}

Maybe @Capstan can confirm this?
I updated StorageExample to take a content type parameter for cp.

[aozarov]

@mziccard I tried to reproduce the content-type requirement using the API Explorer but for me it worked fine without supplying it.

[mziccard]

@aozarov My bad, I was providing a storage object in the request body even though it had no contentType, forgot to add the check. I'm fixing this.

[aozarov]

Looks great, few more minor comments.

[mziccard]

Last comments addressed, thanks @aozarov for the thorough review!