Add support for custom span attributes

[Nancy-Chauhan]

Feature Request\n\nUsers should be able to define custom span attributes with type validation and schema enforcement.\n\n## Motivation\n\nEnterprise customers need to ensure span attributes conform to their internal schemas. Currently attributes are untyped dict[str, Any].\n\n## Proposed Solution\n\n1. Add an AttributeSchema class that defines allowed keys and types\n2. Add an optional schema parameter to Span.__init__\n3. Validate attributes on assignment when schema is set\n\n## Alternatives\n\n- Could use Pydantic validators but that's less flexible\n- Could do validation at export time instead of assignment time