Search: allow excluding posts or groups of posts using the UI rather than code

### Impacted plugin

Search

### What

We can hard code excluding posts, but can't just click a box or add a filter

### How

There are multiple cases where it would be helpful to exclude posts from all search results.

We could also make it easy to import these cases. Things like:
- excluding all posts that are also excluded from robots.txt
- anything that has noindex set - from https://jetpack.com/support/sitemaps/ settings
- various meta created by other plugins
- we use the _jetpack_exclude_from_search post meta to exclude certain post types based on their registered settings

We should figure out the different cases and support better and easier customization.

Examples are from pflv1o-tj-p2

Start by reviewing the existing _jetpack_exclude_from_search post meta and the linked sitemap settings for noindex behavior. The issue also calls out robots.txt exclusions and metadata from other plugins, but does not identify files, tests, or a specific entry point; clarify which cases and UI customization are in scope before implementation.

Written by the indexing model from the issue text.