[entities] Adding Entity Support to Semantic Conventions

[jsuereth]

This is a belated tracking issue for entity support in semantic conventions split from open-telemetry/opentelemetry-specification#4436.

Here is the proposal discussed with semconv + entities groups we're executing against.

Remaining Tasks

• Add entity_associations to signals in Semantic Conventions
• Rename resource group type to entity
• Add support for Descriptive vs. Identifying attributes. (Allow attributes to be identifying or descriptive. weaver#735)
• Update stability requirements for entities