Enable passing additional capability flags

[Weakky]

Overview

Hey, we would like to be able to pass the CapabilityFlags::CLIENT_FOUND_ROWS but all the capabilities are currently hard-coded.

We'd be happy to open a PR but we wanted to make sure you would be ok to have an fn add_capability() somewhere in the API first.

If you're ok with this addition, let us know if you prefer us to do it or if you have the capacity to add this API.

Thank you 🙏