Update semantic conventions for RUM

[pellared]

Related to #242

AFAIK RUM uses OTel Resources which are then send as tags via Zipkin exporter.

From https://github.com/signalfx/gdi-specification/blob/main/specification/semantic_conventions.md

This is the only resource attribute that RUM libraries are supposed to set because it's the only one the Zipkin exporter can understand.

I am not sure it this is true.

Also shouldn't we set splunk.distro.version instead of splunk.rum.version?

@t2t2 @seemk PTAL

PS. Feel free to edit or create a better PR 😉