Support TLS certificates for authenticating against the REST endpoints

[devraj]

Is your feature request related to a problem? Please describe.
You can optionally provide a client generated certificate for authentication against the REST API (see authentication section in the REST API docs).

While considering #65 we should ensure that we have support for TLS certificates.

Describe the solution you'd like

• Investigate if this is compatible with the cloud gateway or is it just a direct client/server configuration
• Outline a mechanism in which we will implement this and how the user will go about using this feature
• Document basic instructions for generating the certificate and key file for user with the command centre

Upon this provide an implementation against this ticket, preferably when the user configures the API client for use.

Describe alternatives you've considered
NA

Additional context

• Update all tests to use this authentication method
• See what is involved in updating our Command Centre