Augment the fixed frequency NCOs by a sweep function

As suggested by @Araneidae in [#21](https://github.com/DiamondLightSource/DLS-MBF/issues/21#issuecomment-2736234053) the two fixed frequency NCOs could be augmented by implementing a sweep function instead. This functionality could be of interest for Elettra (@lonzam), and would be useful for the ESRF as part of the bunch cleaning system.

This requires a change of the driver and gateware, which could become version 1.4.3

I propose to use a very similar implementation of the sweep function used in the sequencer.

The following new PVs would be defined for NCO1:

- (MBF):(AXIS):NCO1:START_FREQ
- (MBF):(AXIS):NCO1:STEP_FREQ
- (MBF):(AXIS):NCO1:END_FREQ
- (MBF):(AXIS):NCO1:COUNT
- (MBF):(AXIS):NCO1:DWELL
- (MBF):(AXIS):NCO1:HOLDHOFF

The former PV will be removed:

- (MBF):(AXIS):NCO1:FREQ

The same will be done for NCO2.

Comments and suggestions are welcome.

edit: add PV `COUNT`