Merge bitcoindevkit/bdk#2005: Add populate_anchor_cache method to bdk

191bdb154e99de616c2409fbfcab3ef5e2257262 feat(electrum): Add `populate_anchor_cache` method (+Sharon)

Pull request description:

  This PR addresses #1982
  It introduces functionality to populate the anchor cache, improving how BDK handles stateful or performance-critical anchor data.

  ### Changes
  - Add `populate_anchor_cache` method to `BdkElectrumClient `in `bdk-core`
  - Improves sync speed by caching anchor transactions in advance
  - Enables pre-caching of anchor transactions to reduce redundant network calls

  Follow-up work will extend this functionality to `bdk-wallet`.

ACKs for top commit:
  evanlinjin:
    ACK 191bdb154e99de616c2409fbfcab3ef5e2257262

Tree-SHA512: b14e9ee5a5deb439951175195419c2e07e513c23a6c242254e610b1f6d38a37cb71f2ae8d82f9c34628acf665126256eab7f37bf9ba6634547e7218433136cbe