Feature Request: allow indexing Jetpack Search without enabling the feature to end users

### Impacted plugin

Search

### What

Allow indexing Jetpack Search before showing it to end users

### How

@georgestephanis pointed out that bigger sites could use an ability to start indexing before enabling instant search to end-users, since bulk indexing can take a while: p1688776149158479/1688755475.233429-slack-C0CMN0V97

Team 51's site had some issues with our Search on blog 141910098, and it was not giving the right results. They had to disable it to revert to regular search, but then they couldn't index it in the background.

One possible solution that I could think of would be to have two separate toggles: `Enable Jetpack Search for admins` and `Enable Jetpack Search for all users`, which would allow admins to start indexing and test it on their site before fully enabling the feature:


This is sort of related to pcNPJE-14J-p2

Start by tracing Jetpack Search's existing settings, indexing flow, and end-user search enablement. Compare the proposed separate admin and all-user toggles with the current behavior. Done means indexing can run in the background while regular end-user search remains available until explicitly enabled.

Written by the indexing model from the issue text.