Introduce OTEL_SEMCONV_EXCEPTION_SIGNAL_OPT_IN

[trask]

Fixes #3363

Introduces environment variable OTEL_SEMCONV_EXCEPTION_SIGNAL_OPT_IN, defined as

Existing instrumentations that record exceptions as span events:

• SHOULD introduce an environment variable OTEL_SEMCONV_EXCEPTION_SIGNAL_OPT_IN supporting the following values:
  • logs - emit exceptions as logs only.
  • logs/dup - emit both span events and logs, allowing for a phased rollout.
  • The default behavior (in the absence of one of these values) is to continue emitting exceptions as span events (existing behavior).
• SHOULD maintain (security patching at a minimum) their existing major version for at least six months after it starts emitting both sets of conventions.
• MAY drop the environment variable in their next major version and emit exceptions as logs only.

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