Echo test endpoint
Summary
    | Data Type | stream | 
    | Data Flow | bidirectional | 
    | Class |  EchoEndpoint | 
    | Built in | YES | 
    | Purpose | test/debug/keepalive | 
    | supported labels | echo | 
Data Structures
None
Sent
"hello"
Responses
"hello"
NOTE
Message size limit is imposed by stack. <4k bytes should always work.
Data is assumed to be a string.