Tektronix AFG3XXX - option to send arbitrary command

As a user using Tektronix AFG devices, I would like the ability to send custom commands to the device and read the responses.

Function generators are complicated devices and sometimes used at short notice. The complexity means it is not necessarily practical to expose *all* of the functionality of the device in nice user-facing records. Providing a mechanism to allow users to send & receive arbitrary strings to the device would give us/scientists more flexibility when using these function generators for 'one-off' experiments.

### Acceptance criteria:
- [ ] There is a PV that can be written to to send a "raw" command straight to the device and read a response (if any)
- [ ] This appears on the OPI

### Notes:
- See TRITON or INSTRON iocs for examples where we already allow arbitrary commands to be sent to devices.