Chain index needs a get_subtree_roots API

### Usage mode

zainod (running the daemon)

### Problem or use case

zallet wants access to subtree roots on first launch, to make sync more efficient. 

### Proposed solution

The blockchain source has them, direct passthrough should be sufficient. 

### Alternatives considered

_No response_

### Area

Indexing / Chain state

### Additional context

_No response_