fix: Add ENDING to allowed shot state on receiving meta information

[jake-arkinstall]

When exit is called by the user program, it:

• Flushes an EXIT message to the result stream
• Tells the runtime that an shot is ending
• pokes the runtime to fetch instructions (e.g. cleanup) emitted as a result of the runtime handling the above
• runs those instructions through the rest of the pipeline
• emits end-of-shot event hook data to the result stream (e.g. INSTRUCTIONLOG, METRICS)
• sends a SELENE:SHOT_END instruction

In the result stream parsing code, receiving an EXIT puts the shot into an ENDING state. This PR allows events through the result stream to be accepted inside of that ENDING state, thus allowing instructionlog and metrics to be collected without error after an exit has been called.