The type of external system (Jira, Linear, ServiceNow, etc.)
Available options:
EXTERNAL_TICKET_SOURCE_TYPE_UNSPECIFIED,
EXTERNAL_TICKET_SOURCE_TYPE_LINEAR,
EXTERNAL_TICKET_SOURCE_TYPE_JIRA,
EXTERNAL_TICKET_SOURCE_TYPE_EMAIL,
EXTERNAL_TICKET_SOURCE_TYPE_CSV_IMPORT,
EXTERNAL_TICKET_SOURCE_TYPE_PUBLIC_API,
EXTERNAL_TICKET_SOURCE_TYPE_FRESHSERVICE,
EXTERNAL_TICKET_SOURCE_TYPE_SLACK,
EXTERNAL_TICKET_SOURCE_TYPE_ZENDESK,
EXTERNAL_TICKET_SOURCE_TYPE_SERVICENOW,
EXTERNAL_TICKET_SOURCE_TYPE_PHONE_CALL,
EXTERNAL_TICKET_SOURCE_TYPE_MICROSOFT