feat(substrate): add remote AWS signer

[lattejed]

Motivation

Currently Substrate only supports local signing, which is a security issue

Solution

Add a signer for nomad-substrate that works with AWS credentials. More specifically, build an AWS singer that implements substrate's Pair keypair / signer trait.

PR Checklist

• [ ] Added Tests
• [ ] Updated Documentation
• [ ] Updated CHANGELOG.md for the appropriate package
• [ ] Ran PR in local/dev/staging