Added Code links to spans as an approved convention

[martinjt]

What are you trying to achieve?
It would be really useful for observability backends to have a consistent way to link to the code that generates spans. This would allow clickable links to span generating code, and therefore aid in debugging.

What did you expect to see?
A new attribute in the Semantic Conventions for a Trace that include a url that links to the code in a cloud system. The suggestion is code.url as the attribute, and guidance that it should be a valid Url.

Additional context.
None