RootCanal: Implement the CSR Vendor command

The CSR Vendor command multiplexes the four
functions read_varid, write_varid, read_pskey, write_pskey
which are being used by the PTS to configure the dongle
before starting the tests.

Bug: 257357422
Test: atest pts-bot:HFP
Change-Id: I5c2ff2db404931e2d4bfb85adad49b80c34b622e