Support Snapcraft advanced grammar in plugin parameters?

### What needs to get done

Not exactly sure as the implementation of advanced grammar seems to be in the Snapcraft project, will that be reusable here?

### Why it needs to get done

Hello, I would like to implement the following Snapcraft part definition:

```yaml
parts:
main:
plugin: cmake
cmake-parameters:
- to armhf:
- -DGGML_VULKAN=OFF
- else:
- -DGGML_VULKAN=ON
```

However this errors with:

```text
$ snapcraft pack --debug
Bad snapcraft.yaml content:
- input should be a valid string (in field 'parts.main.cmake-parameters[10]')
- input should be a valid string (in field 'parts.main.cmake-parameters[11]')
```

as (AFAICT) plugin parameters do not have advanced grammar support.

This feature is especially helpful in Snapcraft as architecture-specific build configurations are quite common.

Start by comparing Snapcraft's advanced grammar implementation with how craft-application validates plugin parameters, using the example in the issue as the input case. Determine whether the grammar can be reused here; done means the conditional cmake-parameters YAML is accepted and architecture-specific values are handled correctly.

Written by the indexing model from the issue text.