Do not allow setting complex params

When setting params in pydantic models we don't want to do `fgs_params.experiment_params = x`, instead we want to go through the pydantic conversion. See (https://github.com/DiamondLightSource/python-artemis/commit/aa05a97bd5808641cd424fc27e2897d49eabc2b3). We should make it so that it's not possible to set these things directly, we can probably do this through `properties`.

## Acceptance Criteria
* Doing `fgs_params.experiment_params = x` is not possible but running it through json is possible
* There are tests