API for access to list object in backend

[MurzNN]

After Keystone's core was reworked by #5665, we miss the ability to access Lists metadata at backend side.

For example, earlier we could get list of fields from needed List via something like this:

const myListFields = context.lists[listName].fieldsByPath;
const myListAuthorFieldType = context.lists[listName].fieldsByPath['author'].adapter.fieldName; // eg 'Relationship'

Now this feature is disappeared, and I can't find any other way to get that information at the backend function.

[MurzNN]

Here https://keystonejs.slack.com/archives/C01STDMEW3S/p1625703474012900?thread_ts=1622762153.345400&cid=C01STDMEW3S is suggested to use this construction:

context.graphql.schema.getType(listKey).getFields()?.[fieldKey].extensions?.keystoneSecretField.compare(secret, hash)

But it not return all needed data about list field (type, database type, etc).

So I fix this via my PR here #6059

[gautamsi]

I need this too, have to make sure the output is formatted based on config