Implement new endpoints for secrets management

[dmjb]

We need some endpoints for interacting with secrets management.

In future, we may want to allow multiple secrets providers to be set up, but for now, we will restrict ourselves to a single secrets manager. Nonetheless, we will include the name default in some of the endpoints to allow us to change

Proposed endpoints:

POST /secrets/                     # Initialize the default secrets provider
GET   /secrets/default          # Get the details of the default provider - specifically, is it encrypted or 1password
GET   /secrets/default/keys # Get the list of keys from the default provider
POST /secrets/default/keys # Create a new secret (encrypted provider only)
PUT   /secrets/default/keys/{key name}     # Update secret value (encrypted provider only)
DELETE /secrets/default/keys/{key name} # Delete the secret (encrypted provider only)