Option to structure Event message format as JSON

[vordimous]

Describe the desired outcome from the user's perspective
As a DevOps observability admin, I want to parse all parts of log messages programmatically. Having a static structure for log messages enables static text parsing methods. Variable parts of log messages should be configurable to output as JSON instead of a flat test string.

Acceptance criteria

• The stdout exporter has the option to print raw text or unformatted JSON.
• The otlp exporter can send the Event message in the body as a flat string or JSON string.
• Use JSON as the default format

Additional context
Open Telemetry Logs Data model body field