attr: Remove Requirement for tftypes Handling from Exported Type/Value Interfaces

### Module version

```
v1.0.0
```

### Use-cases

Provider developers should not need to drop down to the `github.com/hashicorp/terraform-plugin-go/tftypes` type system when creating custom types via `attr.Type` and `attr.Value`. In particular:

`attr.Type`

- `tftypes.AttributePathStepper`
- `TerraformType(context.Context) tftypes.Type`
- `ValueFromTerraform(context.Context, tftypes.Value) (Value, error)`

`attr.Value`

- `ToTerraformValue(context.Context) (tftypes.Value, error)`

The framework recently migrated the framework types into a `types/basetypes` package, which enables embedding in many cases. That package also has framework type system-specific interfaces such as `StringTypable` and `StringValuable` for describing how to convert custom framework types to/from the framework types, e.g.

`basetypes.StringTypable`

- `ValueFromString(context.Context, StringValue) (StringValuable, diag.Diagnostics)`

`basetypes.StringValuable`

- `ToStringValue(ctx context.Context) (StringValue, diag.Diagnostics)`

The work to remove the `tftypes` burden was not completed before version 1.0 though, so custom types developers are still left with this burden.

### Proposal

Ideally, these details of the framework type system `tftypes` handling would be an internal package interface that only the `types/basetypes` types implement. Since version 1.0 is released though, we'll need to be careful about external usage of the existing interface methods, even if the framework internally switches to a different interface.

More thought will be needed on how to accomplish this though, as this type system conversion is fairly central to the framework's data handling.

### References

- #172