**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>

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

This adds support for private docker registries, by adding
an optional `registryAuth` field in the CreateFunctionRequest.
Auth must be passed as base64-encoded basic auth, similar to
how done in Docker file store credentials (~/.docker/config.json).
Credentials are then passed to swarm at service creation.