Support for OTEL_EXPORTER_OTLP_HEADERS/OTEL_EXPORTER_OTLP_{TRACE,LOG,METRICS}_HEADERS

[endocrimes]

Some managed providers require specifying custom headers for managing authentication when shipping telemetry data without a local relaying agent (e.g. Honeycomb) - we should also support that with Spin. (afaict we don't currently plumb those up anywhere when looking in https://github.com/fermyon/spin/blob/ba32722671752321ab7a34b52216559aacacfa51/crates/telemetry/src/env.rs)

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md#specifying-headers-via-environment-variables