Easy-to-read examples

[jpnurmi]

Add small, targeted, easy-to-read examples that demonstrate the most common SDK operations.

Ideas:

• crash.c: captures a crash
• enrich.c: enriches an event with a user, tags, contexts etc.
• feedback.c: captures feedback for an event
• logs.c: sends structured logs with attributes
• message.c: captures a message event
• metrics.c: records counters, gauges, and distribution metrics
• scope.c: captures an event with local scope data
• tracing.c: traces an operation with child spans

With these in place, the existing convoluted example.c could be demoted as a test fixture.

[linear-code]

NATIVE-233