[instr-http] Add (back) dual-emit option for HTTP stable attributes

[JamieDanielson]

• Part of Implement stable http semantic conventions #4897
• See Add option to dual-emit HTTP attributes and HTTP migration plan #5646

Open a new PR for instrumentation-http that re-implements the OTEL_SEMCONV_STABILITY_OPT_IN option for dual-emitting old and new(stable) http attributes.