Limits the upload size of lists to 9 meg size (elastic#72898)

## Summary

Limits the lists to 9 megs upload size so we don't blow up smaller Kibana installs. Users can change/override this using the switch of `xpack.lists.maxImportPayloadBytes` like so:

```
xpack.lists.maxImportPayloadBytes: 40000000
```

That will increase the amount of bytes that can pushed through REST endpoints from 9 megs to something like 40 megs if the end users want to increase the size of their lists and have enough memory in Kibana.

Metrics and suggestions from testing looks like:

```ts
Kibana with 1 gig of memory can upload ~10 megs of a list before possible out of memory issue
Kibana with 2 gig of memory can upload ~20 megs of a list before possible out of memory issue
```  

Things can vary depending on the speed of the uploads of the lists where faster connections to Kibana but slower connections from Kibana to Elastic Search can influence the numbers.  

### Checklist

- [x] [Unit or functional tests](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility) were updated or added to match the most common scenarios

x-pack/plugins/lists/common/constants.mock.ts

@@ -41,7 +41,7 @@ export const OPERATOR = 'included';
 export const ENTRY_VALUE = 'some host name';
 export const MATCH = 'match';
 export const MATCH_ANY = 'match_any';
-export const MAX_IMPORT_PAYLOAD_BYTES = 40000000;
+export const MAX_IMPORT_PAYLOAD_BYTES = 9000000;
 export const IMPORT_BUFFER_SIZE = 1000;
 export const LIST = 'list';
 export const EXISTS = 'exists';

x-pack/plugins/lists/server/config.ts

@@ -11,7 +11,7 @@ export const ConfigSchema = schema.object({
   importBufferSize: schema.number({ defaultValue: 1000, min: 1 }),
   listIndex: schema.string({ defaultValue: '.lists' }),
   listItemIndex: schema.string({ defaultValue: '.items' }),
-  maxImportPayloadBytes: schema.number({ defaultValue: 40000000, min: 1 }),
+  maxImportPayloadBytes: schema.number({ defaultValue: 9000000, min: 1 }),
 });
 
 export type ConfigType = TypeOf<typeof ConfigSchema>;