Allow random list length to be configured by user

**Is your feature request related to a problem? Please describe.**

Arrays produce unpredictable results when a mock is not explicitly provided.

**Describe the solution you'd like**

When arrays are not explicitly given a mock they will default to a length of 2 according to the return value of randomListLength. See https://github.com/ardatan/graphql-tools/blob/99319bea39b070aa4ea5b79f4d0d8c72824bf8a5/packages/mock/src/MockStore.ts#L493

If each entry in the array is an object, this can result in unpredictable results as all the values of the objects will also be auto-filled. It would be really useful to be able to manually configure the mock store to return arrays of length 0.

When calling `addMocksToSchema` an argument could be provided.

`addMocksToSchema({ schema, defaultListLength: 0 })`

**Describe alternatives you've considered**

The alternative is to manually provide a mock of `[]` for all queries you wish to be consistent, but this takes away much of the value of leaving the auto-mocking to handle most of the load.

I am happy to work on this issue but want to gauge interest.

Start in packages/mock/src/MockStore.ts at the linked randomListLength implementation, then trace how addMocksToSchema accepts configuration. Define the requested default list-length option so an unspecified array can produce length 0, and verify that explicit array mocks remain unaffected.

Written by the indexing model from the issue text.