fix(eap-api): Snuba can receive val_double in addition to val_float

[xurui-c]

Sentry sends RPC request containing val_float to Snuba (code here). Snuba receives the request. Snuba then sends response containing val_float to Sentry (code here). Sentry receives the response.

Plan:

1. update the receiving end of both Sentry and Snuba to support both val_double and val_float
2. after some time, replace the val_float in the sending end of both Sentry and Snuba to val_double. This replacement (instead of supporting both) is backward compatible because step 1 ensures that both receiving ends can handle either val_float or val_double
3. after some time, remove val_float from the receiving end of both Sentry and Snuba

Note: "after some time" means all instances of Sentry and Snuba have the updated code

This PR handles the Snuba side of step 1. PR for handling the Sentry side of step 1 is here