chain: SyncRequestBuilderExt/FullScanRequestBuilderExt only work with BlockHash checkpoint data #2437

Describe the enhancement
SyncRequestBuilderExt and FullScanRequestBuilderExt (in bdk_chain::indexer::keychain_txout) are only implemented for builders whose checkpoint data is the default D = BlockHash:

impl<K: Clone + Ord + Debug> SyncRequestBuilderExt<K> for SyncRequestBuilder<(K, u32)> { .. }
impl<K: Clone + Ord + Debug> FullScanRequestBuilderExt<K> for FullScanRequestBuilder<K> { .. }

SyncRequestBuilder<I, D>, FullScanRequestBuilder<K, D>, CheckPoint<D> and LocalChain<D> are all generic over D, but with any other D (e.g. Header) revealed_spks_from_indexer, unused_spks_from_indexer and spks_from_indexer cannot be called:

let tip = CheckPoint::<Header>::new(0, genesis_block(Network::Regtest).header);
let request = SyncRequest::builder_at(0)
    .chain_tip(tip)
    .revealed_spks_from_indexer(&indexer, ..) // error[E0599]: method not found
    .build();

The impls never use the checkpoint data, so they can be made generic over D.

Use case
Building sync/full-scan requests from a KeychainTxOutIndex for a LocalChain<Header> (or any custom checkpoint data), the same way it already works for LocalChain<BlockHash>. This is the same kind of D restriction that #2416 fixed for SyncResponse::is_empty/FullScanResponse::is_empty.

Impact

• Blocking production usage
• Nice-to-have / UX improvement
• Developer experience / maintainability

Are you using BDK in a production project?

• Yes
• No
• Not yet, but planning to

Which backend(s) are relevant (if any)?

• Electrum
• Esplora
• Bitcoin Core RPC
• None / not backend-related (e.g. bdk_chain, bdk_core)
• Other (please specify): ____

Project or organization (optional)

Additional context
Fix: add a D type parameter to both impls. Callers using the default D = BlockHash are unaffected.