Setup dependabot

[dougch]

Security issue notifications

If you discover a potential security issue in s2n we ask that you notify
AWS Security via our vulnerability reporting page. Please do not create a public GitHub issue.

Problem:

For our rust bindings, we have pinned versions for dependencies that really should be checked frequently for updates.

Solution:

GitHub Dependabot automates the dependency update process.

Sibling projects s2n-quic and s2n-netbench are already using dependabot.