What are the key REST API endpoints for triggered sends, contacts, and data extensions?

Triggered Sends: POST /messaging/v1/messageDefinitionSends/{definitionKey}/send — fires a triggered send to one or more subscribers. Body includes To.Address, To.SubscriberKey, and optional subscriberAttributes.
Contacts: POST /contacts/v1/contacts — creates or updates a contact. GET /contacts/v1/contacts/{contactKey} retrieves a contact.
Data Extensions (Async): POST /data/v1/async/dataextensions/{key}/rows — asynchronously upserts rows into a data extension. The async endpoint returns a request token; poll GET /data/v1/async/{requestToken} for status.
Data Extensions (Sync): POST /data/v1/customobjectdata/key/{key}/rowset — synchronous row insert for smaller payloads. Always prefer async for bulk operations.