Add support for routing OTAP and OTLP gRPC exporter traffic through HTTP proxies using the HTTP CONNECT tunneling method. This enables telemetry export from environments behind corporate proxies or firewalls.

This also bring parity with the proxy support in opentelemetry-collector - OTel Collector Exporter Proxy Support. In the Go collector, proxy support is provided automatically through Go's net/http package, which handles HTTP_PROXY environment variables transparently. However, the Rust gRPC library (tonic) does not include built-in proxy support, requiring explicit implementation of the HTTP CONNECT protocol for tunneling gRPC traffic through proxies.