Support referencing attributes across signals in codegen

[lmolkova]

See open-telemetry/opentelemetry-specification#3299.
E.g. user_agent.original is defined within attribute_group in opentelemetry-specification/semantic_conventions folder and referenced in resource semconv here https://github.com/open-telemetry/opentelemetry-specification/blob/main/semantic_conventions/resource/browser.yaml

Code generator allows to specify source folder and would generate all semconv from there, applying --only or --exclude flags to the source. It uses the same subset of files to resolve attributes and generate code for them.

When browser resource semconv is generated, we need to use all resource + cross-signal attributes to resolve attributes, but generate only those that are in resource. there is no such option today.

[lmolkova]

Fixed in #157