Tracking issue for further block editor and full site editing support

Continued from #932 cc @alecgeatches

# Goal

Improve support for the block editor and full site editing by providing blocks to handle two use cases:
1. Display one or more coauthors in a templated manner within post contexts and query loops.
2. Display the queried coauthor in author archive contexts.

# Features

## Blocks
- Author Template
- A repeating pattern that displays multiple coauthors, similar to Post Template within a query loop.
- Wraps around the other blocks listed below to provide them context.
- Author Name
- Choose between Display Name, First Name, Last Name
- Optionally wrapped a link.
- Author Avatar
- Optionally wrapped in a link.
- Author Description

## Block Editor
- Add coauthors to post content.
- Add coauthors to query loops.
- Subscribe to existing CAP store so that a change to the assigned coauthors is reflected in the content.
- Support appearance tools.

## Full Site Editing
- Add coauthors to post templates outside of content.
- Add coauthor data to author archives using an All Authors template type.

## Layouts
- Support the following in the Author Template block
- Separators between authors and a distinct separator for the last author.
- An inline layout typical of bylines.
- A block layout typical of end of article author card.

## Extensibility
- Support the addition of fields in the API.
- Should fields be added using `coauthors_guest_author_fields` or should it go through the REST API framework?
- Support the ability for others to make custom blocks that work in the Author Template block.

# Features in consideration for future versions
- Is it possible to output a Social Icons block and fill it with a coauthor's social links?
- Support more layout options based on user feedback, a grid of cards for example.
- How might we support templates for specific authors in Full Site Editing?

# Approach

## API

Create two API endpoints intended to GET coauthor data.

### GET CoAuthors by `:post-id`

#### Use Case
- Query for the coauthors of a post in post or post template contexts.

#### Request
- Require `:post-id` in URL

#### Response
- Array of CoAuthor objects

#### Authorization
- Intended for editors and up, but to be determined by capability check.
- Plus anyone who is a coauthor of the post in question.

### GET CoAuthor by `:author-name`

#### Use Case
- When rendering a block for a specific coauthor, query for them.
- (future) When editing an author archive template, query for single coauthor.

#### Request
- Require `:author-name` in URL.
- Author term slug without the `cap-` prefix, same as author_name query_var available in author archive requests.

#### Response
- Single CoAuthor object

#### Authorization
- To be determined based on more information from testing full site editing.

## CoAuthor Objects in API Responses

These objects should have the same shape as the objects returned by `get_coauthors()`, with some notable exceptions.

- Author archive URLs should be included.
- Author avatar attachment ids should be included if available.
- Should handle Gravatar as well.

## Developer Experience

Use `wp-scripts` with the ability to run a single build command for all blocks and scripts in CAP.

# Compatibility considerations

- [Social Pack](https://github.com/Automattic/Co-Authors-Plus-Social-Pack)