Implement Secret Management API

[judy2k]

We have a new API for managing account secrets, fully documented here: https://nexmo-developer-staging-pr-579.herokuapp.com/api/account/secret-management

It consists of 4 endpoints to be added to the existing account namespace:

• listSecrets(apiKey) - List the secrets associated with the provided api key.
• getSecret(apiKey, secretId) - Obtain the details of the secret identified by the provided secret ID.
• createSecret(apiKey, newSecret) - Create a secret with the details provided in new secret.
• deleteSecret(apiKey, secretId) - Delete the secret associated with the provided secret ID.

(Please note that the parameter is called account_id in the documentation, but should be called api_key as seen here.)

[RabebOthmani]

FYI @judy2k , I'm closing this issue. The implementation is done. https://github.com/Nexmo/nexmo-dotnet/blob/master/Nexmo.Api/ApiSecret.cs