Implement Sigv4a in pure Ruby

[alextwoods]

Implements sigv4a in pure ruby to ensure sdk users that cannot install the aws-crt gem have access to sigv4a.

See: https://github.com/aws/aws-sdk-ruby-staging/pull/6

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

1. To make sure we include your contribution in the release notes, please make sure to add description entry for your changes in the "unreleased changes" section of the CHANGELOG.md file (at corresponding gem). For the description entry, please make sure it lives in one line and starts with Feature or Issue in the correct format.

2. For generated code changes, please checkout below instructions first:
   https://github.com/aws/aws-sdk-ruby/blob/version-3/CONTRIBUTING.md

Thank you for your contribution!