s3: fix v2 auth failing when path style addressing is off - #9893

Fixes #8520.

I hit the same thing writing an S3-compatible store against a custom endpoint, and came looking to see how rclone handled it.

v2 signs the bucket as part of the canonical resource:

CanonicalizedResource = [ "/" + Bucket ] + <HTTP-Request-URI, from the protocol
name up to the query string> + [ subresource, if present ]

SignHTTP takes that resource straight from req.URL.EscapedPath(). With path style the path is already /bucket/key, so it comes out right by accident. With virtual host style the bucket is in the Host header and the path is only /key, so it never reaches the string to sign and the server computes a different signature.

That matches the report: dropping either --s3-v2-auth or --s3-force-path-style=false fixes it, because one puts the bucket back in the path and the other switches to v4, which signs Host anyway.

This pulls the bucket back out of the Host. With an endpoint configured that is exact; without one it looks for the s3 label, since AWS hosts are bucket.s3.amazonaws.com or bucket.s3.<region>.amazonaws.com.

There were no tests on the v2 signer, so I have added some. The one that matters asserts that the same bucket and key sign identically whether addressed path style or virtual host style, which is the invariant that was broken. Without the fix:

--- FAIL: TestV2SignVirtualHostMatchesPathStyle
    expected: "AWS accesskey:yzu+SH0vG0JLfNfVkIZJazPJxy0="
    actual  : "AWS accesskey:YaxEj2fIy2IYDVmbX6aU1bY1BIA="

What I have not done is test against real AWS, since I do not have a bucket set up for v2 auth to point at. So this is verified against the spec and the unit tests, not end to end. @lewoberst if you are still around, it would be worth a run.

One thing I noticed and left alone: SignHTTP ignores its signingTime argument and uses time.Now(), which is why that test has a retry around the Date header. Happy to do it separately if you want it.

make quicktest passes other than cmd/gitannex and cmd/nfsmount, which fail the same way on a clean checkout here (no rclone on PATH, and mount permissions).