feat(tracing): add langfuse tracing

[yashlamba]

Note: Right now, we are explicitly propagating workflow context. 2 important problems to be fixed:

1. The custom propagate context manager exists to avoid awkward inline imports in the activities. This can be used anywhere. The reason we need it is that we are passing activities around, and the way Temporal sandboxes worker code, langfuse imports don't work (Will open an issue explaining this in detail). The solution to this is to separate activity context/schemas and call them by "name", which Temporal supports.

2. Add maybe a decorator/tracing indicator on activities, so this explicit propagation isn't needed. It is not very clean with how Temporal initialises activities, but we can try.