We've known this would eventually be necessary for a while [1], and
way back in 2017 we started seeing SHA-1 collisions [2].
This patch follows the approach of soft deprecation of SHA1 in tempurl.
It's still a default digest, but we'll start with warning as the
middleware is loaded and exposing any deprecated digests
(if they're still allowed) in /info.
Further, because there is much shared code between formpost and tempurl, this
patch also goes and refactors shared code out into swift.common.digest.
Now that we have a digest, we also move digest related code:
- get_hmac
- extract_digest_and_algorithm
[1] https://www.schneier.com/blog/archives/2012/10/when_will_we_se.html
[2] https://security.googleblog.com/2017/02/announcing-first-sha1-collision.html
Change-Id: I581cadd6bc79e623f1dae071025e4d375254c1d9
2d063cd61f