dash-spv: public header queries on DashSpvClient #1173

Platform reads headers through the storage internals (storage() → block_headers()) for the tip's time and to check an asset lock's block is still on our chain (dashpay/platform#5307).

Add to DashSpvClient:

• tip_header() -> Option<HashedBlockHeader>
• header_at(height) -> Result<Option<HashedBlockHeader>>
• header_height(&BlockHash) -> Result<Option<u32>>

Ok(None) (no header) stays distinct from a read error, as BlockHeaderStorage does.