Authenticate incoming requests from Mattermost

• HTTP apps can request that Mattermost includes a secret-based JWT in each request. See the HTTP Deployment documentation for details.
• AWS apps are invoked via the AWS Lambda Invoke and S3 Get APIs, which are authenticated with “invoke” AWS credentials (access/secret keys). See the AWS Deployment documentation for details.
• OpenFaaS and Kubeless calls and static authentication are still TBD.