Google Content Previews

Google announced that they will allow webmasters to [determine their preferences for content previews on websites](https://webmasters.googleblog.com/2019/09/more-controls-on-search.html) shown in Google search results. The preferences use a `robots` HTML meta tag (or `x-robots-tag` HTTP header) to pass settings along to Googlebot. This appears to be a part of their rich snippets initiative in order to present more relevant data to searchers directly in search results.

#### Describe the solution you'd like
As part of the Jetpack SEO Tools, it would be advantageous to be able to provide Google with expected parameters for this feature. It would need to be able to be set on a per-page level.

Visually, it could be a simple UI add-on to the existing section that currently houses the SEO description in Calypso.


The UI would need 3-4 settings:
1. Block snippets altogether
2. Max snippet length
3. Max video preview length
4. Max image preview size (or block image preview)

Blocking altogether could collapse the other options for a cleaner interface.

#### Additional context
While `data-nosnippet` is also mentioned in the above-linked article, I think this is something to be approached in Gutenberg directly and not appropriate for this feature request, as it would need to touch individual content blocks in the editor.

Start by reviewing the Jetpack SEO Tools integration and the existing SEO description section in Calypso, then confirm how per-page SEO metadata is currently handled. Done means a decided UI and implementation for the four requested Google preview settings, with the resulting robots metadata applied per page.

Written by the indexing model from the issue text.