[opentitanlib,fpga] Allow choosing the JTAG adapter with opentitanlib

### Description

We already have the ability to pass in an `--openocd-adapter-config` to OpenTitanTool to modify the adapter configuration being used. But this is not sufficiently general to be able to be used with e.g. the FTDI adapter (in place of the CMSIS-DAP adapter).

An example patch required for use of the FTDI adapter can be found [here](https://github.com/AlexJones0/opentitan/commit/2b0cb7de6f0810792fad6792803b06123bb30b52). We should implement this, and expose this as some configurable option to users.

Start with OpenTitanTool's handling of --openocd-adapter-config and the adapter setup in opentitanlib. Compare the linked FTDI example patch, then identify how adapter selection should be exposed as a user configuration option. Done means users can select an FTDI adapter as well as the existing CMSIS-DAP adapter.

Written by the indexing model from the issue text.