Create a new session

Requests the creation of a session, to associate future API calls to an end user.

The source_type and source_id fields should be set based on the nature of the end user:

• The source_type should be set to account, and the source_id refer to an account resource ID if the end user is the customer and the account already exists.
• The source_type and source_id are optional and don't need to be set if the end user is a potential customer who does not currently have an account.
• The source_type should be set to person, and the source_id refer to a person resource ID if the end user is the customer and the person already exists.