tfpluginsdk: cannot build schema for terraform type "List of Map of String"

### What happened?
Trying to convert a provider for terraform-provider-auth0 to using TerraformPluginSDK. On `make generate`, i get:
`panic: cannot generate crd for resource auth0_custom_domain: cannot build types for CustomDomain: cannot build the Types for resource "auth0_custom_domain": cannot infer type from schema of field verification: cannot infer type from resource schema of element type of CustomDomain.Verification: cannot infer type from schema of field methods: element type of CustomDomain.Verification.Methods is basic but not one of known basic types`.

The terraform attribute in question: https://github.com/auth0/terraform-provider-auth0/blob/main/internal/auth0/customdomain/resource.go#L89:
```go
"methods": {
Type: schema.TypeList,
Elem: schema.TypeMap,
Computed: true,
Description: "Defines the list of domain verification methods used. ",
},
```

The error comes from https://github.com/crossplane/upjet/blob/main/pkg/types/builder.go#L254, where this case is simply not expected.

I have a hard time figuring out how to deal with this. i guess `types.Universe.Lookup("map[string]string")` is not a thing.
Any advice how to improve upjet to properly handle this?

Start by reproducing the failure with make generate while converting the terraform-provider-auth0 resource. Read pkg/types/builder.go around line 254 and compare it with the methods schema in internal/auth0/customdomain/resource.go around line 89. Done means generation handles the List of Map of String schema without the reported panic.

Written by the indexing model from the issue text.