…tribute interface methods

Reference: #132

As part of upcoming effort to split schema functionality into the `datasource`, `provider`, and `resource` packages, there are some internal implementation details which need to be sorted beforehand. Throughout the 0.x versions, schema functionality has been migrated from the `tfsdk` package into the `internal/fwschema` package, but there is some lingering technical debt of duplicate and confusing methods.

This change creates a new `NestedAttribute` (singular) interface which extends the `Attribute` interface with the methods only required for an attribute that does in fact implement nested attributes. The `tfsdk.Attribute` type is updated to support both the `Attribute` and `NestedAttribute` interfaces as it accurately describes the overloaded abstraction. The `NestedAttributes` (plural) interface remains to describe how `tfsdk.Attribute` implements that concepts under its `Attributes` field. The `NestedAttributes` (plural) interface and its implementations will be removed when `tfsdk.Attribute` is removed.

Another aspect of these changes is cleaning up the type describing methods, removing the separate `Attribute` type `FrameworkType()` method and `NestedAttributes` type `AttributeType()` method.

The upcoming split schemas will implement explicit attribute types which only implement `NestedAttribute` (singular) as appropriate.