Mark classes as internal/final/api as applicable

Problem/Motivation

In order to better signal API we can mark classes/methods that are intended to be API with @api.

For classes/methods we expect to be only be called by TFA we can use @internal.

Any class we do not intend to be extended by others we can mark @fainal

Steps to reproduce

N/A

Proposed resolution

Mark code as applicable.

Remaining tasks

Patch

User interface changes

None

API changes

Clarification of code intended to be API and subject to BC support. Some code that may have been previously used as API may be moved off-api.

Data model changes

None