Failing late when dask-gateway clients passed options are invalid (could been known by a schema)

### How things work

`dask-gateway-server` the server and `dask-gateway` the client can be pre-configured so that the client can send options on how to configure a dask-cluster that the dask-gateway server is to ensure is created.

### Problem situation

A typical case is that one configures dask-gateway-server and client to allow sending details on environment variables. In the following failure, environment variables were something that users could specify via options, but if a user specified them without being string valued they **_errored first at the point in time when workers were created - not when the options were passed_**.

> [...] when specifying `environment` for the dask workers, if any of the values are not strings, it will fail forever without producing a useful error message to the user. The following error message is shown instead:

```
HTTP response headers:
HTTP response body: {"kind":"Status","apiVersion":"v1","metadata":{},"status":"Failure","message":"Pod in version \"v1\" cannot be handled as a Pod: json: cannot unmarshal number into Go struct field EnvVar.spec.containers.env.value of type string","reason":"BadRequest","code":400}
```

### Goals to work towards

To fail earlier when `dask-gateway` client passes config that won't work as can be understood with for example a JSONSchema validation check. Ideally with as little additional complexity introduced as possible in this project as we are very low on maintenance capacity.