arrow

ARROW-12671: [C++] Add OpenTelemetry to ThirdpartyToolchain

As discussed on the mailing list, this is a proof-of-concept that integrates OpenTelemetry for (distributed) tracing. This supports the following:
* OTel is used as a header-only library by default.
* Optionally, Arrow will link to OTel and configure it based on an environment variable.

No other instrumentation is added; this just sets up the build, and then we can consider what to instrument (FlightRPC, query engine, etc.) and also integrate more backends (Jaeger, OTLP, etc.).

Closes #10260 from lidavidm/arrow-opentelemetry

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>