lift the restriction on data and target to be json objects

[dinmukhamedm]

[!IMPORTANT] Make target field in Datapoint optional, update database schema, and adjust frontend components accordingly.

• Behavior:
  • target field in Datapoint struct in datapoints.rs changed from Value to Option<Value>, allowing it to be optional.
  • Updated try_from_raw_value() to handle target as optional.
  • Updated insert_datapoints() and insert_raw_data() in datapoints.rs to accommodate optional target.
  • API route in route.ts updated to accept any JSON value for data and target.
• Database:
  • Altered dataset_datapoints table to make target column nullable in 0002_next_tusk.sql.
  • Added labeling_queue_data table in 0002_next_tusk.sql.
  • Updated schema.ts and relations.ts to reflect new table and nullable target.
• Frontend:
  • Updated manual-add-datapoint-dialog.tsx and export-spans-dialog.tsx to handle optional target and validate JSON inputs.
  • Removed toJsonObject function from export-spans-dialog.tsx as it's no longer needed.

^{This description was created by for e5ccd3f106aa7a1d947628efa8bc298bb52e3cec. It will automatically update as commits are pushed.}