Make transport channel capacity configurable

In [https://github.com/getsentry/sentry-rust/pull/923](), @lorenzocian proposed an improvement to make the transport channel capacity configurable. This could make it less likely that we saturate the capacity of the channel in high-throughput scenarios.

As this change requires us to add a field to ClientOptions, it is breaking, so we decided not to proceed with that PR for now, but it is probably a good improvement to do at some point.