sidecar: make use of RDL-defined types in `Tofino2Sequencer.bsv`

[Aaron-Hartwig]

We're defining them in the RTL which is fragile since we can't rely on codegen to pickup any encoding updates. This should be addressed to use the RDL encoded types so the BSV and Rust can all use the same stuff.

This is blocking oxidecomputer/hubris#2515