feat(spans): send gen_ai spans using the span v2 protocol

[Litarnus]

Adds support for the SpanV2 protocol for gen_ai spans.

When a transaction contains gen_ai spans, they will be serialized in a different format to work around the limitations of the old protocol. The envelope will contain two different items, the first one is the transaction with regular spans and the second one is just gen_ai spans

[cursor]

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[giortzisg]

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