Grid scans: Refactor parameters

In https://github.com/DiamondLightSource/mx-bluesky/pull/1623 I changed the parameter model so that an arbitrary number of grids could be specified. A neater solution here is for the `SpecifiedGrids` class to contain a list of `SingleGrid`s, each of which contains only the information about that one grid. `SpecifiedGrids` should also contain the grid concatenation logic, eg `num_images`, `scan_spec`, `num_grids`.

The parameter structure is a bit clearer this way, and it becomes obvious from the external parameter model as to which bits need to be a list (of the same length) and which bits are a single number. As of the above PR we use runtime validation to check that the right bits are the same length.

See https://github.com/DiamondLightSource/mx-bluesky/pull/1623#discussion_r2793508093 for initial discussion

## Acceptance Criteria
- Refactor above complete