Make SigningInstructions public (#2730)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
I'm building an s3 server in rust and I need signature verification to
work from the server side when receiving requests. To do that the server
needs to generate a signature and then compare it to the one that the
client has already sent along. I believe that the sign module in
aws-sigv4 http_request contains the functions required to do that.

<!--- If it fixes an open issue, please link to the issue here -->

## Description
<!--- Describe your changes in detail -->
Exported the `SigningInstructions` struct in the sign module of
http_request for aws-sigv4.

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->


_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

---------

Co-authored-by: Chris Holcombe <chholcombe@tesla.com>
Co-authored-by: John DiSanti <jdisanti@amazon.com>