Add callback validation and error handling

[yaythomas]

Context

Validate callback tokens and operation existence

Technical Requirements

• Validate callback tokens and operation existence
• Add appropriate error responses for invalid callbacks

Acceptance Criteria

• Callback tokens and operation existence validated
• Appropriate error responses added for invalid callbacks

[bchampp]

I think we need the callback token encoding to have two parts:

1. Execution ARN -- which execution is the callback token for,
2. Operation ID -- which operation in the history is the callback response targeting? (in the case of multiple callbacks)

[bchampp]

I opened a PR to standardize the request handling for callbacks - I'll pick this up since it's in the same turf.

PR: #59