Implement Span Streaming API

[Litarnus]

This issue serves as a collection of necessary steps that need to be done for implementing the streaming API

• [X] Implement JSON serialization
• [ ] Apply attributes from scope to spans
• [ ] Add trace_lifecycle option to switch between modes
• [ ] Add before_send_spans callback option
• [ ] set propagation context
• [ ] Ensure startSpan behaves correctly
• [ ] (Optional) Create own workflow for Spans instead of using applyToEvent

[linear[bot]]

PHP-44