Consider adding schema definition string method on types

In the _terraform-plugin-codegen-framework_ code strings for the types for use in generated schema are produced along the following lines:

```go
switch {
case v.Bool != nil:
if v.Bool.CustomType != nil {
aTypes.WriteString(v.Bool.CustomType.Type)
} else {
aTypes.WriteString("types.BoolType")
}
// ...
}
```

A method could be created for each of the types so that the logic for determining which string to return would be come the responsibility of the type itself rather than the caller.

For instance:

```go
switch {
case v.Bool != nil:
aTypes.WriteString(v.Bool.SchemaString)
// ...
}
```