`sentry_options_set_traces_sampler` is missing a user data argument

[jpnurmi]

Unlike other Native SDK callbacks sentry_options_set_traces_sampler doesn’t provide a user_data parameter which normally acts as a closure allowing access to the subsystem implementation within the handler.

Originally posted by @tustanivsky in getsentry/sentry-unreal#1057 (comment)

[linear]

NATIVE-114 `sentry_options_set_traces_sampler` is missing a user data argument

[JoshuaMoelans]

iirc we didn't add user_data as a parameter since a traces sampler callback function itself already takes in a custom_sampling_context which can, in a sense, hold user data; this is not a strong argument not to add it though, as the user_data could just be passed in when setting the callback, whereas the custom_sampling_context needs to manually be passed in with each call to sentry_transaction_start(tx_ctx, custom_sampling_ctx).

I've opened up #1346