Introduce `OTEL_SEMCONV_EXCEPTION_SIGNAL_OPT_IN`

[trask]

To support transitioning from recording exceptions as span events to recording them as logs.

OTEL_SEMCONV_EXCEPTION_SIGNAL_OPT_IN=logs   # emit logs only
OTEL_SEMCONV_EXCEPTION_SIGNAL_OPT_IN=both   # emit both span events and logs
# unset = span events only (current default behavior)

This would allow instrumentations to start adopting log-based exceptions in their current version under this opt-in flag, with the plan to change their default behavior (and drop support for OTEL_SEMCONV_EXCEPTION_SIGNAL_OPT_IN) in their next major version bump (see https://github.com/open-telemetry/opentelemetry-specification/blob/main/oteps/4430-span-event-api-deprecation-plan.md#per-instrumentation).

Even after instrumentations start emitting exceptions as logs, users will still have options to route those as span events, see https://github.com/open-telemetry/opentelemetry-specification/blob/main/oteps/4430-span-event-api-deprecation-plan.md#sending-log-based-exceptions-and-events-as-span-events

Prototype: open-telemetry/opentelemetry-java-instrumentation#16049