Implement Span Streaming API

[Litarnus]

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

• 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
• Migrate contexts to attributes
• startTransaction should be a noop when using span streaming mode
• [ ]

[linear]

PHP-44