Add `body_digest` option to `sign_v4/10`

[wojtekmach]

An example where this is useful is: https://github.com/wojtekmach/req/pull/301 :)

[onno-vos-dev]

Will review, merge and release after work (in a couple hours) assuming everything looks good 👌

[onno-vos-dev]

Will release shortly 👌

[onno-vos-dev]

@jonatanklosko @wojtekmach I'll try to bring the release train PR https://github.com/aws-beam/aws_signature/pull/24 along in this change. So bear with me if it won't work right away, I'm on it 👌

[onno-vos-dev]

🎉 https://hex.pm/packages/aws_signature/0.3.2

Thanks @jonatanklosko for the review and @wojtekmach for the PRs!

[wojtekmach]

Thanks all, I really appreciate it!