kad: make `automatic_bootstrap_interval` publicly configurable

### Description

I would like `automatic_bootstrap_interval` to be made publicly configurable.

### Motivation

 The default 500ms timeout seems too short, it consumes a lot more resources than before.

### Current Implementation

`set_automatic_bootstrap_throttle` exists but not publicly exported, it's for tests only.

### Are you planning to do it yourself in a pull request ?

Yes