Fix the type of `id` on type `BaseItem`, to be type `unknown`, not `string`

[dcousens]

This pull request updates the type of id on type BaseItem to be of type unknown instead of string. This helps reduce a number of type errors where string | number | null is possible, but only string was accepted by the BaseItem.

This is not strictly related to https://github.com/keystonejs/keystone/issues/8846, but will likely be required as part of fixing that problem in any event.

[codesandbox-ci[bot]]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 6d7547e88c4d841e132e993cf0acac5a73034179:

Sandbox	Source
@keystone-6/sandbox	Configuration