Log Based Registry: Annotation

Associated with proposal #25

We need to decide how arbitrary metadata referred to as Annotations (see [prior discussion](https://github.com/bytecodealliance/SIG-Registries/discussions/11)) will be represented in the log.

Annotations will be their own entry type, and will contain an "annotation type" field and either arbitrary data or a hash to it.

## Questions

#### A. Where are annotations stored?
1. Annotations are stored directly in the log as part of the main hash chain.
* **Advantages**
* Annotations are placed at the log head and can "see" authorization changes directly.
* The log remains a single chain.
2. Annotations are stored as part of a per-release log that branches off from the release entry.
* **Advantages**
* Annotations do not contribute to the main hash chain length.
3. ?

#### B. How is authority to create annotations controlled?
1. Authority to create an annotation is granted by a role and lasts till removed. The role may need to be parameterized to enable maintainers/admins to grant only certain annotation rights (e.g. `annotator:alert` granting only annotation-type `alert`).
* **Advantages**
* Does not require a new entry type.
2. Authority to create an annotation is granted by a special entry which specifies what annotations can be created and how many can be made.
* **Advantages**
* Gives much more granular control.
3. ?