Consider mapping for use with associated_external_type

Currently, the generation of `To<...>()/From<...>()` methods (refer to [Consider handling of associated_external_type for all types](https://github.com/hashicorp/terraform-plugin-codegen-framework/issues/50#top)) uses a direct mapping from the Terraform Plugin Framework types to the API SDK types.

For example, usage of `associated_external_type` in the following specification results in the generation of the `To<...>()/From<...>()` methods.

```json
{
"datasources": [
{
"name": "example",
"schema": {
"attributes": [
{
"name": "list_nested_attribute",
"list_nested": {
"nested_object": {
"associated_external_type": {
"import": {
"path": "example.com/apisdk"
},
"type": "*apisdk.Type"
},
"attributes": [
{
"name": "int64_attribute",
"int64": {
"computed_optional_required": "optional"
}
}
]
},
"computed_optional_required": "optional"
}
}
]
}
}
],
"provider": {
"name": "example"
}
}
```

```go
func (v ListNestedAttributeValue) ToApisdkType(ctx context.Context) (*apisdk.Type, diag.Diagnostics) {
var diags diag.Diagnostics

if v.IsNull() {
return nil, diags
}

if v.IsUnknown() {
diags.Append(diag.NewErrorDiagnostic(
"ListNestedAttributeValue Value Is Unknown",
`"ListNestedAttributeValue" is unknown.`,
))

return nil, diags
}

return &apisdk.Type{
Int64Attribute: v.Int64Attribute.ValueInt64Pointer(),
}, diags
}

func (v ListNestedAttributeValue) FromApisdkType(ctx context.Context, apiObject *apisdk.Type) (ListNestedAttributeValue, diag.Diagnostics) {
var diags diag.Diagnostics

if apiObject == nil {
return NewListNestedAttributeValueNull(), diags
}

return ListNestedAttributeValue{
Int64Attribute: types.Int64PointerValue(apiObject.Int64Attribute),
state: attr.ValueStateKnown,
}, diags
}
```

The following assumptions are made during the generation of the `To<...>()/From<...>()` methods based on the specified `associated_external_type`:

- The API SDK type is a struct with fields which are named with the camel-cased attribute name (i.e., `int64_attribute` => `Int64Attribute`).
- The values held within the API SDK type are pointer values of the attribute value (i.e., `int64` => `*int64`).

### Proposal

Additional flexibility could be introduced by allowing the specification of "mapping" between the Terraform Plugin Framework types and the types specified by `associated_external_type`. This would increase the usefulness of the generated `To<...>()/From<...>()` methods and accommodate cases where a direct 1-2-1 mapping between the Terraform Plugin Framework types and the API SDK types was not present.