Add a new .../:feature/variants API
This adds
- `GET /api/admin/projects/:projectId/features/:featureName/variants` which returns
```json
{ version: '1', variants: IVariant[] }
```
- `PATCH /api/admin/projects/:projectId/features/:featureName/variants` which accepts a json patch set and updates the feature's variants field and then returns
```json
{ version: '1', variants: IVariant[] }
```
- `PUT /api/admin/projects/:projectId/features/:featureName/variants`
which accepts a IVariant[] and overwrites the current variants list for the feature defined in :featureName and returns
```json
{ version: '1', variants: IVariant[] }
- This also makes sure the total weight of all variants is == 1000
- Validates that there is at least 1 'variable' variant if there are variants
- Validates that 'fix' variants total weight can't exceed 1000
- Adds tests for all these invariants.
Co-authored-by: Simon Hornby <simon@getunleash.ai>
