**What**
- During function creation, accept an array of strings defining swarm secrets
that are required for the service
- Update docs
- Add new guide on using the secrets capability
- Add new sample function to highlight using environment variables
- Update `ApiKeyProtected` sample function to utilize the new secrets
capabilities
**Why**
- This allows secrets to remain encrypted at rest instead of being unencrypted
in environment variables and yaml files.
Fixes#285
Signed-off-by: Lucas Roesler <lucas.roesler@gmail.com>
