Add workflows to support Messaging and API contracts

[erinknight242]

A user might want to use Totem for both testing messaging and API endpoints. This is the first step toward fully supporting those separate use cases.

When creating a new contract, a user should choose if they are creating a Messaging contract or API contract (Add New button could be a dropdown button, listing the workflow options). If they select messaging, their contract form will have the current fields (namespace, type, etc) and prepopulate the contract with the ID and Timestamp properties. The fields will still not be required until we add the feature to support workflow configuration (#62).

Something to consider, right now we are setting up these two predefined workflows. In the future though, a user might want to create their own workflows (#63). They could have two services, and the defaults for the messaging in one could be different from the defaults for the other, so they might want a separate workflow for each of those messaging types. We do not need to support this on this ticket, but the back-end design should take into account that there could be more than one workflow of each data type in the future.

Acceptance Criteria:

• A user can create a messaging contract that shows the default values
• A user can create an API contract that does not have any default values
• Saving a contract should validate against the rules for that contract's workflow
• Testing a message should still function as it currently does, regardless of the workflow used to define the contract