add support for per-signal urls

[tatchi]

This PR adds support for per-signal URLs configuration. See https://opentelemetry.io/docs/specs/otel/protocol/exporter/#endpoint-urls-for-otlphttp

I've only modified the ocurl client for now to reduce diff. Once approved, I can make the same changes for the lwt client

[tatchi]

Thanks for the review, I've applied the same changes to the cohttp-lwt lib

[c-cube]

Thank you :)