feat: sem_conv stability mode

[hannahramadan]

Lays the foundation for accepting a new environment variable, OTEL_SEMCONV_STABILITY_OPT_IN, which gives users the ability to determine which semantic conventions to use. Individual instrumentation are expected to add metrics/attributes by calling the methods present in OpenTelemetry::SemanticConventions::StabilityMode. This borrows from Pythons implementation.

Only HTTP is supported for now (spec) and the values defined so far are:

• http - emit the new, stable HTTP and networking conventions only
• http/dup - emit both the old and the stable HTTP and networking conventions
• default - continue emitting whatever version of the old experimental HTTP and networking conventions the instrumentation was emitting previously.
• Note: http/dup has higher precedence than http in case both values are present

[kaylareopelle]

Link to instrumentation that uses this PR: https://github.com/open-telemetry/opentelemetry-ruby-contrib/pull/1166

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